Dangerous and cancer-causing properties of products and chemicals in the oil refining and petrochemical industry--Part II: Carcinogenicity, mutagenicity, and developmental toxicity of 1,3-butadiene

DNA modifications: Biomarkers for the exposome?

The concept of the exposome is the encompassing of all the environmental exposures, both exogenous and endogenous, across the life course. Many, if not all, of these exposures can result in the generation of reactive species, and/or the modulation of cellular processes, that can lead to a breadth of modifications of DNA, the nature of which may be used to infer their origin. Because of their role in cell function, such modifications have been associated with various major human diseases, including cancer, and so their assessment is crucial. Historically, most methods have been able to only measure one or a few DNA modifications at a time, limiting the information available. With the development of DNA adductomics, which aims to determine the totality of DNA modifications, a far more comprehensive picture of the DNA adduct burden can be gained. Importantly, DNA adductomics can facilitate a "top-down" investigative approach whereby patterns of adducts may be used to trace and identify the originating exposure source. This, together with other 'omic approaches, represents a major tool for unraveling the complexities of the exposome and hence allow a better a understanding of the environmental origins of disease.

Duration-response association between occupational exposure and pancreatic cancer risk: meta-analysis

BACKGROUND

Evidence is lacking on the occupational exposure time window to chemical agents related to pancreatic cancer risk.

AIMS

This study performed meta-regression and meta-analysis to examine the dose-response association between occupational exposure duration to chemical agents and pancreatic cancer risk.

METHODS

We searched and reviewed studies on exposure duration and pancreatic cancer in five databases (Cochrane Library, EMBASE, PubMed, ScienceDirect and Web of Science) from inception to 16 May 2022. Exposure refers to the years a worker was exposed to any chemical agent, and outcome variables were pancreatic cancer incidence and mortality.

RESULTS

We identified 31 studies, including 288 389 participants. In the meta-regression, the positive dose-response association indicated pancreatic cancer risk increased slightly with every additional year of exposure duration (slope = 1.01; 95% confidence interval [CI] 1.00-1.02). Pancreatic cancer risk increased with an exposure duration of 1-10 (relative risk [RR] = 1.04; 95% CI 1.02-1.06), 11-20 (RR = 1.11; 95% CI 1.05-1.16), and 21-30 years (RR = 1.39; 95% CI 1.12-1.73).

CONCLUSIONS

Pancreatic cancer risk increased as occupational exposure duration increased, with an exposure time window ranging from 1 to 30 years.

Determining the endocrine disruption potential of industrial chemicals using an integrative approach: Public databases, in vitro exposure, and modeling receptor interactions

Environmental and occupational exposure to industrial chemicals has been linked to toxic and carcinogenic effects in animal models and human studies. However, current toxicology testing does not thoroughly explore the endocrine disrupting effects of industrial chemicals, which may have low dose effects not predicted when determining the limit of toxicity. The objective of this study was to evaluate the endocrine disrupting potential of a broad range of chemicals used in the petrochemical sector. Therefore, 139 chemicals were classified for reproductive toxicity based on the United Nations Globally Harmonized System for hazard classification. These chemicals were evaluated in PubMed for reported endocrine disrupting activity, and their endocrine disrupting potential was estimated by identifying chemicals with active nuclear receptor endpoints publicly available databases. Evaluation of ToxCast data suggested that these chemicals preferentially alter the activity of the estrogen receptor (ER). Four chemicals were prioritized for in vitro testing using the ER-positive, immortalized human uterine Ishikawa cell line and a range of concentrations below the reported limit of toxicity in humans. We found that 2,6-di-tert-butyl-p-cresol (BHT) and diethanolamine (DEA) repressed the basal expression of estrogen-responsive genes PGR, NPPC, and GREB1 in Ishikawa cells, while tetrachloroethylene (PCE) and 2,2'-methyliminodiethanol (MDEA) induced the expression of these genes. Furthermore, low-dose combinations of PCE and MDEA produced additive effects. All four chemicals interfered with estradiol-mediated induction of PGR, NPPC, and GREB1. Molecular docking demonstrated that these chemicals could bind to the ligand binding site of ERα, suggesting the potential for direct stimulatory or inhibitory effects. We found that these chemicals altered rates of proliferation and regulated the expression of cell proliferation associated genes. These findings demonstrate previously unappreciated endocrine disrupting effects and underscore the importance of testing the endocrine disrupting potential of chemicals in the future to better understand their potential to impact public health.

Epoxide adducts at the guanine residue within single-stranded DNA chains: reactivity and stability studies

Emphasis was placed in this work on the assessment of structural and biological features of nucleobase adducts that result from the reaction of DNA with epoxide derivatives. Thus we have prepared and characterized a set of site-specifically modified oligonucleotides at N7-position of a guanine residue, upon reaction with diepoxibutane, with the purpose of further investigating some of their biochemical features. The stability of the lesion-containing DNA fragments has also been investigated and clearly shows that the latter modified oligomers may be used as substrates for in vitro enzymatic assays, aimed at determining the biological effects within cell of these chemically induced DNA damage.

Flanking sequences modulate diepoxide and mustard cross-linking efficiencies at the 5'-GNC site

Diepoxybutane, diepoxyoctane, and mechlorethamine are cytotoxic agents that induce interstrand cross-links between the N7 positions of deoxyguanosine residues on opposite strands of the DNA duplex preferentially at 5'-GNC sequences. We have systematically varied the identity of either the base 5' to the cross-linked deoxyguanosine residues or the intervening base pair to determine flanking sequence effects on cross-linking efficiency. We used synthetic DNA oligomers containing four 5'-N(1)GN(2)C sites that varied either N(1) or N(2). Interstrand cross-links were purified through denaturing polyacrylamide gel electrophoresis and then subjected to piperidine cleavage. The amount of cleavage at each deoxyguanosine residue, representative of cross-linking efficiency at that site, was determined by sequencing gel analysis. Our data suggest that cross-linking efficiency varies with the identity of N(1) similarly (purines > pyrimidines) for diepoxybutane, diepoxyoctane, and mechlorethamine but that the effects of N(2) differ for the three compounds.

DNA interstrand cross-linking by a mycotoxic diepoxide

The diepoxide mycotoxin (2R, 3R, 8R, 9R)-4,6-decadiyne-2,3:8,9-diepoxy-1,10-diol (repandiol) was both isolated from the mushroom Hydnum repandum and synthesized de novo. Repandiol was found to form interstrand cross-links within a restriction fragment of DNA, linking deoxyguanosines on opposite strands primarily within the 5'-GNC and 5'-GNNC sequences preferred by diepoxyoctane. However, repandiol was a significantly less efficient cross-linker than either of the diepoxyalkanes (diepoxyoctane and diepoxybutane) to which it was compared.

Mutagenic spectrum of butadiene-derived N1-deoxyinosine adducts and N6,N6-deoxyadenosine intrastrand cross-links in mammalian cells

Reactive metabolites of 1,3-butadiene, including 1,2-epoxy-3-butene (BDO), 1,2:3,4-diepoxybutane (BDO(2)), and 3,4-epoxy-1,2-butanediol (BDE), form both stable and unstable base adducts in DNA and have been implicated in producing genotoxic effects in rodents and human cells. N1 deoxyadenosine adducts are unstable and can undergo either hydrolytic deamination to yield N1 deoxyinosine adducts or Dimroth rearrangement to yield N(6) adducts. The dominant point mutation observed at AT sites in both in vivo and in vitro mutagenesis studies using BD and its epoxides has been A --> T transversions followed by A --> G transitions. To understand which of the butadiene adducts are responsible for mutations at AT sites, the present study focuses on the N1 deoxyinosine adduct at C2 of BDO and N(6),N(6)-deoxyadenosine intrastrand cross-links derived from BDO(2). These lesions were incorporated site-specifically and stereospecifically into oligodeoxynucleotides which were engineered into mammalian shuttle vectors for replication bypass and mutational analyses in COS-7 cells. Replication of DNAs containing the R,R-BDO(2) intrastrand cross-link between N(6) positions of deoxyadenosine yielded a high frequency (59%) of single base substitutions at the 3' adducted base, while 19% mutagenesis was detected using the S,S-diastereomer. Comparable studies using the R- and S-diastereomers of the N1 deoxyinosine adduct gave rise to approximately 50 and 80% A --> G transitions with overall mutagenic frequencies of 59 and 90%, respectively. Collectively, these data establish a molecular basis for A --> G transitions that are observed following in vivo and in vitro exposures to BD and its epoxides, but fail to reveal the source of the A --> T transversions that are the dominant point mutation.

Diepoxybutane and diepoxyoctane interstrand cross-linking of the 5S DNA nucleosomal core particle

Diepoxyalkanes form interstrand cross-links in DNA oligomers preferentially at 5'-GNC sites. We have examined cross-linking by 1,2,3,4-diepoxybutane (DEB) and 1,2,7,8-diepoxyoctane (DEO) within a fragment of the 5S RNA gene of Xenopus borealis in both the free and nucleosomal states. Sites and efficiencies of interstrand cross-linking were probed through denaturing polyacrylamide gel electrophoresis and quantitative phosphorimagery. Both agents targeted 5'-GNC sites for cross-linking in the restriction fragment in its free state, and DEO also targeted 5'-GNNC sites. Monoalkylation occurred at all deoxyguanosines. The sites for both monoalkylation and interstrand cross-linking were similar in nucleosomal and free DNA, and cross-linked DNA was cleanly incorporated into the core particle structure. These findings suggest that the 5S core particle is able to tolerate any structural abnormalities induced by diepoxide cross-linking.

Metabolism and toxicity of 2-methylpropene (isobutene)--a review

2-Methylpropene (MP) or isobutene is a gaseous chemical used on a large scale in the synthetic rubber industry. The present review covers the rather scarce literature on MP with respect to its metabolic fate and toxicity in laboratory animals and humans. It has been shown both in vivo and in vitro that MP is metabolized to the primary metabolite 2-methyl-1,2-epoxypropane (MEP) by rodent and human liver tissue. The formation of this reactive epoxide intermediate is catalyzed by CYP2E1, while epoxide hydrolase and glutathione S-transferase appear to be involved in its inactivation. In rats, the capacity to absorb and metabolize MP is saturable. MP is oxidized to compounds that are mainly excreted in urine. Data indicate that rodents can tolerate low levels of MP without apparent toxicity. The primary metabolite MEP, however, is able to produce genetic damage in both prokaryotic and eukaryotic cells in vitro. MP is thus not toxic per se but elicits metabolic activation to become potentially harmful. Consequently, the balance between formation and detoxification of MEP plays a key role in determining the potential toxicity of the parent compound. Obviously, further research, including repeated exposure toxicity studies, is required before an estimation of the risk for man can be made.

Dangerous and cancer-causing properties of products and chemicals in the oil-refining and petrochemical industry--Part XXII: Health hazards from exposure to gasoline containing methyl tertiary butyl ether: study of New Jersey residents

Methyl tertiary butyl ether has caused the following cancers in rats and mice: kidney, testicular, liver, lymphomas, and leukemias. Thus, in the absence of adequate data on humans, it is biologically plausible and prudent to regard methyl tertiary butyl ether-for which there is sufficient evidence of carcinogenicity in experimental animals-as a probable human carcinogen. This means that some humans are at extreme risk of contracting cancers resulting from their exposure to oxygenated gasoline containing methyl tertiary butyl ether. Immediately after the introduction of methyl tertiary butyl ether into gasoline, many consumers of this product in New Jersey, New York, Alaska, Maine, Pennsylvania, Colorado, Arizona, Montana, Massachusetts, California, and other areas, experienced a variety of neurotoxic, allergic, and respiratory illnesses. These illnesses were similar to those suffered by refinery workers from the Oil, Chemical, and Atomic Workers Union who mixed methyl tertiary butyl ether with gasoline. Additionally, these illnesses occurred following exposure to extremely low levels of methyl tertiary butyl ether in gasoline, particularly when compared to the adverse health effects that occurred only after exposure to very high levels of conventional gasoline. Thus, gasoline containing methyl tertiary butyl ether exhibited substantially more toxicity in humans than gasoline without this additive. A number of oil industry-sponsored or influenced reports alleged that these illnesses were either unrelated to exposure to reformulated gasoline or were characteristic of some yet-to-be-identified communicable disease. These studies further alleged that the widespread concern was not about illness, but was merely a reaction to the odor and the five cent increase in the price of gasoline. To clarify the significance of this issue, it is important to note that consumers have been using gasoline for many decades, with complaints only occurring following exposure to high levels at 100s ppm or higher. After the introduction of methyl tertiary butyl ether gasoline there were thousands of human health complaints. The sudden increase in widespread illnesses from which many thousands of individuals throughout the United States began to suffer immediately following the introduction of methyl tertiary butyl ether into gasoline provides strong and unquestionable evidence that gasoline containing methyl tertiary butyl ether is associated with human illnesses. When considering the severity of the illnesses in humans, it is prudent that this highly dangerous chemical be promptly removed from gasoline and comprehensive studies be conducted to assess the long-term effects that human may experience in the future from past and current exposure.

1,2,5,6-Diepoxyhexane and 1,2,7,8-diepoxyoctane cross-link duplex DNA at 5'-GNC sequences

The carcinogenicity of epoxide compounds has been attributed to covalent binding to DNA. Whereas monoepoxides form only monoadducts, diepoxides can form both monoadducts and interstrand cross-links. The latter are believed to be the more significant cytotoxic lesions as diepoxides are frequently more carcinogenic and mutagenic than their monoepoxide analogues. We therefore examined the relative DNA interstrand cross-linking capabilities of several diepoxides with respect to chain length, molecular flexibility, reported carcinogenic potential, and DNA sequences targeted. Using denaturing polyacrylamide gel electrophoresis, we found that 1,2,5,6-diepoxyhexane and 1,2,7,8-diepoxyoctane share the 5'-GNC target sequence previously found for 1,2,3,4-diepoxybutane [Millard, J.T., and White, M.M. (1993) Biochemistry 32, 2120-2124] and that the efficiency of cross-linking this sequence may reflect carcinogenicity, 1,2,5,6-Diepoxycyclooctane, the biologically inactive rigid analogue of 1,2,5,6-diepoxyhexane, was found to be a poor cross-linker of all DNA sequences examined. Moreover, increasing the diepoxyalkane chain length did not result in enhanced cross-linking ability.

Figuring out cancer

Cancer continues to increase throughout the industrial world. Aging of the population, smoking habits, and improvements in diagnosis do not account completely for these patterns. About 5 percent of cancers occur in persons under age 45, while 65 percent of all cancers occur in persons over age 65. For this older age group, many of their most common tumors, in addition to those linked with smoking, are not curable and appear to be increasing in incidence as well as mortality in a number of industrial countries. Evaluating cancer causes by looking at an age-adjusted rate obscures important differences that are occurring at different age groups. In order to reduce the cancer burden further, it is important to look for bona fide explanations of recent shifts in cancer patterns, especially those not related to cigarette smoking or other habits.