chemical carcinogens definition

carcinogen definition: 1. a substance that causes cancer: 2. a substance that causes cancer: 3. a substance that can…. If it is a two-step process, then the second abnormality that occurs in this intermediate cell population will lead to a malignancy. Cleaver (Cleaver et al., 1975) identified that individuals with xeroderma pigmentosum (XP) had a markedly increased rate of development of skin cancers in response to sun exposure. In its bound form, the α2u-globulin-d-limonene epoxide cannot be readily degraded so that there is an accumulation in lysosomes and ultimately an increase in cell death and consequent regeneration leading to tumors. One of the most extensively studied metabolic systems influencing activation and inactivation has been acetylation of aromatic amines (Hein, 2006). These, unfortunately, are not definitive but provide considerable evidence that can be taken into account in an overall weight of evidence evaluation for a chemical. Cellular processes of differentiation and carcinogenesis. This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes. First, the initiating carcinogens in tobacco which act as reactive electrophiles (having electron deficient atoms) react with nucleophilic (electron rich) sites in the cell. This is because many of these assays are in vitro and involve considerable levels of cytotoxicity. The basis for this long latency has posed a theoretical challenge to scientists continuing to today, but the necessity for multiple specific genetic errors occurring stochastically in a single cell provides the explanation for this latency (see below). Just because you had contact with a … (C) Every time a normal stem cell (SN) replicates, a mistake can occur in a critical part of a gene essential for converting the cell ultimately into a malignant cell. An individual chemical may be further distinguished: Examples of initiating carcinogens include compounds such as polycyclic hydrocarbons and nitrosamines, biological agents, certain viruses, and physical agents such as X-rays and UV light. However, even identifying the causes does not guarantee that we are able to implement preventive measures, as can be all too readily seen with cigarette smoking, obesity, and sexually transmitted diseases. However, specific, targeted knockout, and transgenic models can be extremely useful in evaluating specific modes of action. Based on the research over the past five decades, it is clear that the immunosurveillance is not of the malignant cells themselves but of the infections that can increase the risk of certain tumors. In humans, with respect to bladder cancer, acetylation of the aromatic amines is actually a deactivation process. This leads to a direct mitogenic stimulus of the rat thyroid follicular cells and ultimately the development of benign and malignant tumors. Toxicology began with the simple premise that all substances are poisons, but the dose makes the poison (Paracelsus, see Gallo, 2008). This population can be clonally expanded, as described in the initiation-promotion model, but this does not have to occur. In humans, examples also have been identified. Asbestos and tobacco products are examples of carcinogens. Repetitive Traumatic Brain Injury Is Associated With TDP-43 Alterations, Neurodegeneration, and Glial Activation in Mice. Threshold and non-threshold chemical carcinogens: A survey of the present regulatory landscape. The effect of estrogen on these target tissues is to increase the number of DNA replications in the target tissue pluripotential stem cell population (Preston-Martin et al., 1990). Certain viruses can also act as carcinogens by interacting with DNA. N and λ are assumed to be constant, and cell proliferation rates are also assumed to be constant. A plethora of genotoxicity assays have been developed during the past four decades, which were designed not only as a means of identifying a mechanism by which a chemical can produce cancer but also as a short-term screening test for identifying chemical carcinogens (Kirkland et al., 2007). Aromatic amine acetylation dimorphism was actually the first of the differences identified, which led to the development of the field which now is commonly referred to as molecular epidemiology (Lower and Bryan, 1973). To reduce cancer, the reverse of this must occur. 60–68, Increased cell division as a cause of human cancer, The role of transgenic mouse models in carcinogen identification, Time-course comparison of xenobiotic activators of CAR and PPARalpha in mouse liver, Incorporating human dosimetry and exposure into high-throughput in vitro toxicity screening, Gap-junction communication in chemical carcinogenesis, Overview: Using mode of action and life stage information to evaluate the human relevance of animal toxicity data, A compilation of two decades of mutagenicity test results with the Ames Salmonella typhimurium and L5178Y mouse lymphoma cell mutation assays, Meat intake, heterocyclic amine exposure, and metabolizing enzyme polymorphisms in relation to colorectal polyp risk, IPCS conceptual framework for evaluating a mode of action for chemical carcinogenesis, The initiation and growth of tumors. Transgenic and knockout models have generally not been widely used for screening purposes, and mode of action analysis if a positivity is detected in these assays is a difficult exercise. Also, DNA replication by itself can lead to a “spontaneous” alteration in the DNA, effectively acting to produce initiation. An increase in proliferation in those cells that are committed to differentiation or even fully differentiated will not lead to an increase in cancer risk. In the liver, the tumor effect was due only to the interaction of 2-AAF with the DNA of the normal hepatocytes, with this interaction occurring much less frequently in the intermediate cell population (foci cells) (Cohen and Ellwein, 1990b). Activities to further develop this framework are continuing by incorporating various aspects of dose-response, exposure, cumulative risk, and other aspects involved in overall risk assessment. This has included in vitro inhibition of gap junction communication (Ruch and Trosko, 2001) as well as specific in vivo assays that utilize a combination of DNA-reactive and proliferative stimuli, such as the Ito’s medium-term assay (Ito et al., 2003). Formula derived by Armitage and Doll (1954) to describe the exponential increase in incidences of tumors with age. … They also involve indirect effects on DNA and chromosomes rather than a direct interaction with DNA. Samuel M. Cohen, Lora L. Arnold, Chemical Carcinogenesis, Toxicological Sciences, Volume 120, Issue suppl_1, March 2011, Pages S76–S92, The malignant potential in these tumors appears to occur with the initial transformation of the cell without an intervening benign proliferation. It has been concluded that this mode of action in rats is not relevant to humans, based predominantly on a quantitative assessment of the process but also involving some qualitative issues. Extrapolating estimate of risk in humans for DNA-reactive carcinogens has traditionally assumed a linear, nonthreshold dose-response based to a large degree on the radiation model of mutagenesis and carcinogenesis (Calabrese, 2009) forming the basis for regulation of such chemicals. Administration to monkeys beginning at birth and continuing for up to 24 years also did not increase cancer risk. However, having two rare events occur in the same cell during normal development would be an extraordinarily rare event; approximately one individual per million births would be expected to develop such an abnormality. Other test systems have been developed to try to identify the chemicals acting to increase carcinogenic risk by non–DNA-reactive processes. Oxford University Press is a department of the University of Oxford. The carcinogens in them are caused by the radiation they release, such as gamma rays or alpha particles.. CERCLA says that all radionuclides are carcinogens. During DNA replication in the pluripotential cells, whether under normal circumstances generating one daughter cell and a committed cell or whether duplicating itself into two pluripotential cells, mistakes can occur in the DNA, which are fixed permanently and inherited by the cells (Fig. Examples include the observation by Rehn (1895) in 1895 that workers in the aniline dye industry in Germany frequently developed bladder cancer and more recent observations concerning the induction of angiosarcomas in patients exposed to contrast material used for radiologic imaging studies (Vajdic et al., 1986) and vinyl chloride exposure in the workplace in Louisville, Kentucky (Creech and Johnson, 1974). Identification and definition of chemical carcinogens: review of criteria and research needs. A decrease in cell deaths will lead to an accumulation of cells. 3). It is particularly easy to be seduced by the spectacular technologies now available, losing sight of the basic biology. In contrast to rodents, where liver is the most common target site for carcinogenicity of chemicals, liver cancer in humans is generally not related to chemical exposure except in unusual instances such as aflatoxin and ethanol. If there is injury, this is the way tissues are repaired and how this population can be replenished. A physiological role for flavin-containing monooxygenase (FMO3) in humans? Genotoxins cause irreversible genetic damage or mutations by binding to DNA. However, he posed the question as to why individuals who had inherited this disorder did not have all their retinoblasts develop into tumors. Mutation and cancer: a model for human carcinogenesis, Correlation of structural class with no-observed-effect levels: a proposal for establishing a threshold of concern, Co-carcinogenicity of sodium saccharin and N-[4-(5-nitro-2-furyl)-2-thiazolyl] formamide for the urinary bladder, Toxicity Testing in the 21st Century: A Vision and a Strategy, Predictive toxicogenomics approaches reveal underlying molecular mechanisms of nongenotoxic carcinogenicity, Methods of DNA adduct determination and their application to testing compounds for genotoxicity, DNA adduct formation and removal in specific liver cell populations during chronic dietary administration of 2-acetylaminofluorene, Chirurgical Observations Relative to the Cataract. The Armitage-Doll model predicts an exponential increase of tumor incidence with age, such as seen for cancers of the colon, prostate, urinary bladder, lung, and others. Many non–DNA-reactive carcinogens have a mode of action involving interaction with specific cellular receptors, including nuclear receptors. Another example is Hodgkin's disease in the United States (Watanabe et al., 1986). Fortunately, there are many talented scientists investigating these issues, and progress is being made. It is thus clear that chloroform is carcinogenic only when the exposure is sufficiently high to produce cytotoxicity. Childhood tumors, such as retinoblastoma, occur virtually exclusively in childhood. In animal models, hormones and various treatments that affect the endocrine system frequently lead to an increased risk of tumors in the target population. If two steps are required, this second error produces a malignant stem cell (SM), which also can duplicate itself or undergo commitment (CM) to differentiation (DM). In addition, these must occur in a pluripotential (tissue stem) cell, and the cell must replicate to fix the DNA alteration produced by a DNA adduct as a permanent mutation. Sodium saccharin in the rat leads to pronounced alterations in the composition of the various normal urinary constituents, leading to the formation of calcium phosphate–containing precipitate. 2-Naphthylamine is metabolically activated to a reactive electrophile and is mutagenic and carcinogenic. High doses of d-limonene produce an increased risk of renal tubular tumors in male rats (Hard and Whysner, 1994; IARC WG, 1999). However, there is frequently information in humans that is highly relevant to the overall mode of action, even if chemical-specific data are not available. The actual toxicity assessment for chloroform is based on its cytotoxicity effects on the liver and kidney, not on the carcinogenicity, which is essentially a bystander effect resulting from the cytotoxicity. In contrast, in individuals developing retinoblastomas on a sporadic basis, a spontaneous error had to occur in both alleles during normal growth and development during cell replication of the retinoblast. Fundamental to this model is that the transitions from normal to intermediate to malignancy occur during DNA replication, each with a certain probability that is usually very low. There was no apparent increase in cell proliferation in either population compared with untreated controls. Distinction between classes of chemicals based on their ability to generate DNA reactivity is the basis for the classification of chemical carcinogens to this day and forms the basis for the distinction of potential risks to humans in regulatory decision making. Additionally, acquired immunodeficiency disease syndrome (AIDS) is associated with an increased risk of developing cancers (Grulich et al., 2007). Furthermore, this model requires the presence of an intermediate clonal expansion of initiated cells for the development of the ultimate malignancy. One of these assumptions is that the rate of cell proliferation in a given tissue remains constant throughout lifetime, and another is that the number of target cells remains constant throughout the lifetime. This includes tumors of the rat thyroid, the rat testicular Leydig cells, and the endocrine cells of the stomach (which lead to carcinoid tumors). If more than two steps are required, there will be additional intermediate cell populations. Saffiotti U. These three chemicals all act by increasing cytotoxicity and regeneration. The role of metabolic activation in carcinogenesis and the importance of DNA damage and mutation have led to additional avenues of research regarding mechanisms of carcinogenesis and influences on the carcinogenic process. Increasing knowledge of the multiple steps of carcinogenesis is leading to improved methods for screening chemicals for carcinogenic activity and for regulatory decision making. These include B-cell lymphomas secondary to Epstein-Barr virus infections (or secondary to regeneration itself of the B-cell population), squamous cell tumors secondary to human papilloma virus, or Kaposi's sarcoma secondary to human herpes virus-8. Erianthridin Induces Non-small Cell Lung Cancer Cell Apoptosis through the Suppression of Extracellular Signal-regulated Kinase Activity. Although the model of initiation and promotion has served the carcinogenesis community well with respect to investigating the multistep nature of carcinogenesis and the carcinogenic properties of chemicals, there actually are several difficulties with this model (Cohen, 1998a, 1998b; Cohen and Ellwein, 1991). Ultimately, the ability of a chemical to be DNA reactive can be demonstrated by an evaluation of DNA adduct formation either in vitro or, better, in vivo. Ashby J. Once the final alteration occurs and malignancy develops, in contrast to what has long been held as true for malignancies, not all these malignant cells will replicate and some of them can yield fully differentiated cells. Based on previous screening methodologies, it is very likely that screening of the liver will be useful in detecting rodent liver carcinogens but probably will not be of much use in detecting carcinogens active at other target sites. This can only be gained by a shorter term assessment of specific key events that are likely to occur in a postulated mode of action. Although these genotoxicity assays, specifically DNA reactivity assays, have been a major development in chemical carcinogenesis, both with respect to determination of mechanism and for screening new chemicals for potential genotoxic activity, it was already apparent at the time when many of these assays were being developed in the 1970’s that there were a large number of chemicals that produced cancer in animal models (as well as in humans) that were not positive in any of these assays. chemical carcinogenesis The induction of malignancy by a known or putative chemical carcinogen, which can be occupational (e.g., aromatic amines, arsenic, benzene, cadmium, chromium ores, soots, tars, vinyl chloride), environmental (e.g., aflatoxin, asbestos, tobacco) or iatrogenic (e.g., alkylating agents, anabolic steroids, phenacetin). Based on the multistage model of carcinogenesis described above and the framework for mode of action and human relevance evaluation, another system has recently been proposed that utilizes shorter term exposure, assessing specifically the considerations for DNA reactivity, cell proliferation, estrogenicity, and immunosuppression (Cohen, 2004, 2010). However, a more restrictive definition of genotoxicity is to limit the effects to DNA reactivity specifically. ... looking in particular at the definitions and methodologies used to derive . Again, the tumors arise from a process that leads to cell proliferation, and this process occurs early in the overall carcinogenicity of these chemicals. If a dose of the initiator was fractionated into multiple parts but the cumulative amount administered remained the same, similar tumor incidences and numbers were induced. 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