Comparison of the mutations induced by p-benzoquinone, a benzene metabolite, in human and mouse cells

Generation, degradation mechanism, and toxicity evaluation of pigmented compounds in Leucosceptrum canum nectar

Leucosceptrum canum nectar (LCN) emerges as a novel food resource, distinguished by its unique dark brown hue. This study delves into the composition and toxicity assessment of novel pigments within LCN. Through liquid chromatography-tandem mass spectrometry (LC-MS/MS) and chemical synthesis, seventeen 2,5-di-(N-(-)-prolyl)-para-benzoquinone (DPBQ) analogs in LCN were identified. These compounds are synthesized in LCN via the Michael addition reaction, utilizing p-benzoquinone (BQ), derived from phenol metabolism, and amino acids as substrates in an alkaline environment (pH = 8.47 ± 0.06) facilitated by dissolved ammonia and the presence of alkaloids. Analytical techniques, including principal component analysis (PCA), orthogonal partial least squares discrimination analysis (OPLS-DA), and volcano plot analysis, were employed to investigate DPBQ analog degradation within the nectar and honey's unique environments. Toxicity assays revealed that DPBQ analogs exhibited no toxicity, displaying a significant difference in toxicity compared to the precursor compound BQ at concentrations exceeding 25 μM.

p-Benzoquinone initiates non-invasive urothelial cancer through aberrant tyrosine phosphorylation of EGFR, MAP kinase activation and cell cycle deregulation: Prevention by vitamin C

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p-Benzoquinone induces non-invasive urothelial carcinoma in a guinea pig model.

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The mechanisms involved are persistent growth signaling and cell cycle deregulation.

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Vitamin C prevents p-benzoquinone-induced non-invasive urothelial carcinoma.

According to WHO classification system, non-invasive urothelial carcinoma represents urothelial carcinoma in situ (CIS) and dysplasia. Dysplastic urothelium often progresses to CIS that further advances to urothelial carcinoma (UC). The strongest risk factor for UC is cigarette smoking. However, the pathogenesis of cigarette smoke (CS)-induced UC is poorly understood. Earlier we had shown that p-benzoquinone (p-BQ), a major toxic quinone derived from p-benzosemiquinone of CS in vivo, is a causative factor for various CS-induced diseases. Here, using a guinea pig model we showed that prolonged treatment with p-BQ led to non-invasive UC, specifically carcinoma in situ (CIS) of the renal pelvis and dysplasia in the ureter and bladder. The mechanisms of carcinogenesis were p-BQ-induced oxidative damage and apoptosis that were later suppressed and followed by activation of epidermal growth factor receptor, aberrant phosphorylation of intracellular tyrosine residues, activation of MAP kinase pathway and persistent growth signaling. This was accompanied by deregulation of cell cycle as shown by marked decrease in the expression of p21^waf1/cip1 and cyclin D1 proteins as well as hyperphosphorylation of pRb. UC has been characterised by histopathology and immunohistochemistry showing aberrant CK20, increased Ki-67, and marked p53 nuclear immunopositivity with uniformly negative labelling of CD44. Oral supplementation of vitamin C (30 mg/kg body weight/day) prevented CIS of the renal pelvis and dysplasia in the ureter and bladder. Since majority of non-invasive UC progresses to invasive cancer with increased risk of mortality, our preclinical study might help to devise effective strategies for early intervention of the disease.

Molecular fingerprints of environmental carcinogens in human cancer

Identification of specific molecular changes (fingerprints) is important to identify cancer etiology. Exploitable biomarkers are related to DNA, epigenetics, and proteins. DNA adducts are the turning point between environmental exposures and biological damage. DNA mutational fingerprints are induced by carcinogens in tumor suppressor and oncogenes. In an epigenetic domain, methylation changes occurs in specific genes for arsenic, benzene, chromium, and cigarette smoke. Alteration of specific microRNA has been reported for environmental carcinogens. Benzo(a)pyrene, cadmium, coal, and wood dust hits specific heat-shock proteins and metalloproteases. The multiple analysis of these biomarkers provides information on the carcinogenic mechanisms activated by exposure to environmental carcinogens.

Catalytic ozone oxidation of benzene at low temperature over MnOx/Al-SBA-16 catalyst

The low-temperature catalytic ozone oxidation of benzene was investigated. In this study, Al-SBA-16 (Si/Al = 20) that has a three-dimensional cubic Im3m structure and a high specific surface area was used for catalytic ozone oxidation for the first time. Two different Mn precursors, i.e., Mn acetate and Mn nitrate, were used to synthesize Mn-impregnated Al-SBA-16 catalysts. The characteristics of these two catalysts were investigated by instrumental analyses using the Brunauer-Emmett-Teller method, X-ray diffraction, X-ray photoelectron spectroscopy, and temperature-programmed reduction. A higher catalytic activity was exhibited when Mn acetate was used as the Mn precursor, which is attributed to high Mn dispersion and a high degree of reduction of Mn oxides formed by Mn acetate than those formed by Mn nitrate.

Ortho-quinones of benzene and estrogens induce hyperproliferation of human peripheral blood mononuclear cells

Benzene is a known leukemogen. It has been hypothesized that benzene and natural estrogens initiate cancer by forming ortho-quinones (catechol quinones) that react with DNA in cells. These quinones form depurinating DNA adducts that generate the mutations leading to cancer. This study examined whether the treatment of normal human peripheral blood mononuclear cells with the ortho-quinones of benzene or estradiol would form DNA adducts and elicit an alteration in the proliferation of these cells. Both estradiol-3,4-quinone and benzene ortho-quinone formed depurinating DNA adducts and significantly increased the mitogen-induced proliferation of normal blood mononuclear cells. Immunophenotyping of the estradiol-3,4-quinone-treated blood cells indicated that monocyte/macrophage, natural killer and T-cells were particularly prone to hyperproliferation. Thus, DNA damage induced by the ortho-quinones of benzene and estradiol may promote the growth of human blood mononuclear cells, including those that appear in large numbers in leukemia and lymphoma.

Mutation of cysteine residue 455 to alanine in human topoisomerase IIalpha confers hypersensitivity to quinones: enhancing DNA scission by closing the N-terminal protein gate

Several quinone-based metabolites of industrial and environmental toxins are potent topoisomerase II poisons. These compounds act by adducting the protein, and previous studies suggest that they increase levels of enzyme-associated DNA strand breaks by at least two potential mechanisms. Quinones act directly on the DNA cleavage-ligation equilibrium of topoisomerase II by inhibiting the rate of ligation. They also block the N-terminal gate of the protein, thereby stabilizing topoisomerase II in its "closed clamp" form and trapping DNA in the central annulus of the enzyme. It has been proposed that this latter activity enhances DNA cleavage by increasing the population of enzyme molecules with DNA in their active sites, but a causal relationship has not been established. In order to more fully characterize the mechanistic basis for quinone action against topoisomerase II, the present study characterized the sensitivity of human topoisomerase IIalpha carrying a Cys455-->Ala mutation (top2alphaC455A) toward quinones. Cys455 was identified as a site of quinone adduction by mass spectrometry. The mutant enzyme was approximately 1.5-2-fold hypersensitive to 1,4-benzoquinone and the polychlorinated biphenyl quinone 4'Cl-2,5pQ, but it displayed wild-type sensitivity to traditional topoisomerase II poisons. The ability of 1,4-benzoquinone to inhibit DNA ligation mediated by top2alphaC455A was similar to that of wild-type topoisomerase IIalpha. However, the quinone induced approximately 3 times the level of clamp closure with the mutant enzyme. These findings strongly support the hypothesis that the ability of quinones to block the N-terminal gate of the type II enzyme contributes to their actions as topoisomerase II poisons.

Quinone-induced enhancement of DNA cleavage by human topoisomerase IIalpha: adduction of cysteine residues 392 and 405

Several quinone-based metabolites of drugs and environmental toxins are potent topoisomerase II poisons. These compounds act by adducting the protein and appear to increase levels of enzyme-DNA cleavage complexes by at least two potentially independent mechanisms. Treatment of topoisomerase IIalpha with quinones inhibits DNA religation and blocks the N-terminal gate of the protein by cross-linking its two protomer subunits. It is not known whether these two effects result from adduction of quinone to the same amino acid residue(s) in topoisomerase IIalpha or whether they are mediated by modification of separate residues. Therefore, this study identified amino acid residues in human topoisomerase IIalpha that are modified by quinones and determined their role in the actions of these compounds as topoisomerase II poisons. Four cysteine residues were identified by mass spectrometry as sites of quinone adduction: Cys170, Cys392, Cys405, and Cys455. Mutations (Cys --> Ala) were individually generated at each position. Only mutations at Cys392 or Cys405 reduced sensitivity ( approximately 50% resistance) to benzoquinone. Top2alphaC392A and top2alphaC405A displayed faster rates ( approximately 2-fold) of DNA religation than wild-type topoisomerase IIalpha in the presence of the quinone. In contrast, as determined by DNA binding, protein clamp closing, and protomer cross-linking experiments, mutations at Cys392 and Cys405 did not affect the ability of benzoquinone to block the N-terminal gate of topoisomerase IIalpha. These findings indicate that adduction of Cys392 and Cys405 is important for the actions of quinones against the enzyme and increases levels of cleavage complexes primarily by inhibiting DNA religation.

The p-benzoquinone DNA adducts derived from benzene are highly mutagenic

Benzene is a human leukemia carcinogen, resulting from its cellular metabolism. A major benzene metabolite is p-benzoquinone (pBQ), which can damage DNA by forming the exocyclic base adducts pBQ-dC, pBQ-dA, and pBQ-dG in vitro. To gain insights into the role of pBQ in benzene genotoxicity, we examined in vitro translesion synthesis and in vivo mutagenesis of these pBQ adducts. Purified REV1 and Polkappa were essentially incapable of translesion synthesis in response to the pBQ adducts. Opposite pBQ-dA and pBQ-dC, purified human Poliota was capable of error-prone nucleotide insertion, but was unable to perform extension synthesis. Error-prone translesion synthesis was observed with Poleta. However, DNA synthesis largely stopped opposite the lesion. Consistent with in vitro results, replication of site-specifically damaged plasmids was strongly inhibited by pBQ adducts in yeast cells, which depended on both Polzeta and Poleta. In wild-type cells, the majority of translesion products were deletions at the site of damage, accounting for 91%, 90%, and 76% for pBQ-dA, pBQ-dG, and pBQ-dC, respectively. These results show that the pBQ-dC, pBQ-dA, and pBQ-dG adducts are strong blocking lesions, and are highly mutagenic by predominantly inducing deletion mutations. These results are consistent with the lesion structures predicted by molecular dynamics simulation. Our results led to the following model. Translesion synthesis normally occurs by directly copying the lesion site through base insertion and extension synthesis. When the lesion becomes incompatible in accommodating a base opposite the lesion in DNA, translesion synthesis occurs by a less efficient lesion loop-out mechanism, resulting in avoiding copying the damaged base and leading to deletion.

Comparison of the mutagenic activity of the benzene metabolites, hydroquinone and para-benzoquinone in the supF forward mutation assay: a role for minor DNA adducts formed from hydroquinone in benzene mutagenicity

Benzene, a ubiquitous environmental pollutant and occupational hazardous chemical, is a recognised human leukaemogen and rodent carcinogen. The mechanism by which benzene exerts its carcinogenic effects is to date unknown but it is considered that mutations induced by benzene-DNA adducts may play a role. The benzene metabolite, para-benzoquinone (p-BQ) following reaction in vitro with DNA, forms four major adducts, which include two adducts on 2'-deoxyguanosine 3'-monophosphate (dGp). Reaction of DNA with the benzene metabolite hydroquinone (HQ) results in only one major DNA adduct, which corresponds to one of the dGp adducts formed following reaction with p-BQ. The mutagenicity of the adducts formed from these two benzene metabolites was investigated using the supF forward mutation assay. Metabolite-treated plasmid (pSP189) containing the supF gene was replicated in human Ad293 cells before being screened in indicator bacteria. Treatment with 5-20 mM p-BQ gave a 12 to 40-fold increase in mutation rate compared to 5-20 mM HQ treatment, a result reflected in the level of DNA modification observed (8 to 26-fold increase compared to HQ treatment). Treatment with p-BQ gave equal numbers of GC --> TA transversions and GC --> AT transitions, whereas treatment with HQ gave predominantly GC-->AT transitions. The spectra of mutations achieved for the two individual treatments were shown to be significantly different (P = 0.004). A combination of both treatments also resulted in a high level of GC --> AT transitions and a synergistic increase in the number of multiple mutations, which again predominated as GC --> AT transitions. Sites of mutational hotspots were observed for both individual treatments and one mutational hotspot was observed in the multiple mutations for the combined treatment. These results suggest that the dGp adducts formed from benzene metabolite treatment may play an important role in the mutagenicity and myelotoxicity of benzene.

Dysregulation of apoptosis by benzene metabolites and their relationships with carcinogenesis

Benzene is a widely recognized human carcinogen, the effect of which is attributed to the production of reactive oxygen species (ROS) from its metabolites. Although there have been many reports on the relationship between DNA damage induced by benzene metabolites and carcinogenesis, only a report approached the subject by examining the benzene-induced dysregulation of apoptosis. Inhibition of apoptosis, aberrantly prolonging cell survival, may contribute to cancer by facilitating the insurgence of mutations and by creating a permissive environment for genetic instability. In this study, we examined the mechanism of antiapoptotic effects by benzene metabolites, p-benzoquinone (BQ) and hydroquinone (HQ), and their relationships with carcinogenesis. BQ and HQ inhibited the apoptotic death of NIH3T3 cells induced by both serum starvation and lack of an extracellular matrix (ECM). An antioxidant agent, N-acetylcysteine, significantly inhibited the antiapoptotic effects induced by benzene metabolites, indicating that the effects were mainly due to the production of ROS. Furthermore, BQ and HQ inhibited the in vitro caspase-3 activation, suggesting that the inhibition of caspase-3 activation due to ROS produced by BQ- and HQ-treatment was related to the suppression of apoptosis. The cells that escaped apoptosis could survive with the addition of serum and attachment to the ECM. Levels of 8-oxo-7,8-dihydro-2'-deoxyguanosine were higher in the cells which survived after BQ- and HQ-treatment than in the normal cells. Furthermore, the cells treated with BQ and HQ showed greater proliferation than normal cells under low-serum conditions and anchorage-independent growth in soft agar. These findings suggested that benzene metabolites induced dysregulation of apoptosis due to caspase-3 inhibition, which contributes to carcinogenesis.

Exploring environmental causes of altered ras effects: fragmentation plus integration?

Mutations in ras genes are the most common abnormality of oncogenes in human cancer and a major example of activation by point mutation. Experimental and epidemiological studies support the notion that Ki-ras activation and expression may be chemically related. We discuss the potential role of several environmental compounds in the induction or promotion of ras mutations in humans, with a focus on exocrine pancreatic cancer, the human tumor with the highest prevalence at diagnosis of Ki-ras mutations. Organochlorine compounds, organic solvents, and coffee compounds may play an indirect role in causing Ki-ras mutations, rather than as direct inducers of the mutations. Although for some organochlorine compounds the induction of point mutations in ras oncogenes cannot be excluded, it seems more likely that the effects of these compounds are mediated through nongenomic or indirectly genotoxic mechanisms of action. Organic solvents also may act via enzymatic induction of ras mutagens or by providing a proliferation advantage to ras-mutated cell clones. In exocrine pancreatic cancer, caffeine, other coffee compounds, or other factors with which coffee drinking is associated could modulate Ki-ras activation by interfering with DNA repair, cell-cycle checkpoints, and apoptosis. Asbestos, cigarette smoking, and some dietary factors also may be involved in the initiation or the promotion of Ki-ras mutations in lung and colon cancers. Further development of the mechanistic scenarios proposed here could contribute to a meaningful integration of biological, clinical, and environmental knowledge on the causes of altered ras effects.

Assessing DNA damage and health risk using biomarkers

Carcinogenesis is a multi-stage and prolonged process. At the present time, our knowledge of biological activities along the process is incomplete, therefore, a variety of experimental data are used to assess health risk from exposure to environmental chemicals. However, experimental approaches may not be adequate unless human data are available to support the assessment. In this brief review, benzene (CAS No. 71-43-2), a well-established human leukemogen, will be used as an example to illustrate the challenge in assessing toxicological mechanisms and cancer risk. Benzene has been shown to form DNA-adducts in experimental animals but the adducts have proved elusive of detection in human. Several toxic metabolites of benzene have been identified but the metabolite(s) responsible for the carcinogenic activities is unknown. Furthermore, the significant differences between rodents and human in response to benzene exposure are not understood. Therefore, the bone marrow specificity for the induction of leukemia in human by benzene remains to be elucidated. These complications illustrate the complexity of the assessment process and identify serious information gaps. These information gaps can be viewed as research opportunities to provide more precise data for assessment of toxicological effects and health risk.

Detection of oxidative species and low-molecular-weight DNA in skin following dermal exposure with JP-8 jet fuel

Dermal absorption of JP-8 jet fuel can lead to skin irritation within hours after exposure. This study detected the formation of oxidative species and low-molecular-weight DNA in rat skin as potential indicators of JP-8-induced skin injury. At 0, 1, 2, 4 and 6 h after the beginning of a 1-h exposure, skin samples were removed and analyzed for oxidative species formation and low-molecular-weight DNA analysis. At 1, 2 and 4 h, mean oxidative species levels increased significantly (P < 0.05) above unexposed samples. Significantly higher (P < 0.05) low-molecular-weight DNA values were observed at 4 and 6 h compared with unexposed controls. These results demonstrate significant increases in oxidative species and low-molecular-weight DNA levels in the skin following dermal exposure to JP-8. These responses may serve as indicators of skin injury following exposure to JP-8 jet fuel and other volatile chemicals or mixtures.