Exaggerated carcinogenicity of chemicals

Animal models and conserved processes

BACKGROUND

The concept of conserved processes presents unique opportunities for using nonhuman animal models in biomedical research. However, the concept must be examined in the context that humans and nonhuman animals are evolved, complex, adaptive systems. Given that nonhuman animals are examples of living systems that are differently complex from humans, what does the existence of a conserved gene or process imply for inter-species extrapolation?

METHODS

We surveyed the literature including philosophy of science, biological complexity, conserved processes, evolutionary biology, comparative medicine, anti-neoplastic agents, inhalational anesthetics, and drug development journals in order to determine the value of nonhuman animal models when studying conserved processes.

RESULTS

Evolution through natural selection has employed components and processes both to produce the same outcomes among species but also to generate different functions and traits. Many genes and processes are conserved, but new combinations of these processes or different regulation of the genes involved in these processes have resulted in unique organisms. Further, there is a hierarchy of organization in complex living systems. At some levels, the components are simple systems that can be analyzed by mathematics or the physical sciences, while at other levels the system cannot be fully analyzed by reducing it to a physical system. The study of complex living systems must alternate between focusing on the parts and examining the intact whole organism while taking into account the connections between the two. Systems biology aims for this holism. We examined the actions of inhalational anesthetic agents and anti-neoplastic agents in order to address what the characteristics of complex living systems imply for inter-species extrapolation of traits and responses related to conserved processes.

CONCLUSION

We conclude that even the presence of conserved processes is insufficient for inter-species extrapolation when the trait or response being studied is located at higher levels of organization, is in a different module, or is influenced by other modules. However, when the examination of the conserved process occurs at the same level of organization or in the same module, and hence is subject to study solely by reductionism, then extrapolation is possible.

Animal models in an age of personalized medicine

Personalized medicine is based on intraspecies differences. It is axiomatic that small differences in genetic make-up can result in dramatic differences in response to drugs or disease. To express this in more general terms: in any given complex system, small changes in initial conditions can result in dramatically different outcomes. Despite human variability and intraspecies variation in other species, nonhuman species are still the primary model for ascertaining data for humans. We call this practice into question and conclude that human-based research should be the primary means for obtaining data about human diseases and responses to drugs.

Cadmium-induced cancers in animals and in humans

Discovered in the early 1800s, the use of cadmium and various cadmium salts started to become industrially important near the close of the 19th century, rapidly thereafter began to flourish, yet has diminished more recently. Most cadmium used in the United States is a byproduct from the smelting of zinc, lead, or copper ores, and is used to manufacture batteries. Carcinogenic activity of cadmium was discovered first in animals and only subsequently in humans. Cadmium and cadmium compounds have been classified as known human carcinogens by the International Agency for Research on Cancer and the National Toxicology Program based on epidemiologic studies showing a causal association with lung cancer, and possibly prostate cancer, and studies in experimental animals, demonstrating that cadmium causes tumors at multiple tissue sites, by various routes of exposure, and in several species and strains. Epidemiologic studies published since these evaluations suggest that cadmium is also associated with cancers of the breast, kidney, pancreas, and urinary bladder. The basic metal cationic portion of cadmium is responsible for both toxic and carcinogenic activity, and the mechanism of carcinogenicity appears to be multifactorial. Available information about the carcinogenicity of cadmium and cadmium compounds is reviewed, evaluated, and discussed.

Transgenic assays for mutations and cancer: current status and future perspectives

Transgenic mouse modelling has proved to be a powerful approach to explore the various steps involved in spontaneous and induced carcinogenesis. Some of the multitude of models currently available have the potential to become a substitute for the expensive, long-term rodent bioassay to predict carcinogenicity of environmental compounds. Here, we review the progress in the development and use of transgenic mouse models specifically for the purpose of carcinogenicity and mutagenicity testing.

Thiophilic ring-opening and rearrangement reactions of epoxyketone natural products

Thiol additions to the highly functionalized core structures of aranorosin- and manumycin-type antibiotics reveal the general reactivity patterns of epoxyketone natural products. Rapid hemiacetal and hydrate formations decrease the reactivity of the epoxyketone moiety in aqueous media toward the cellular scavenger glutathione, and secondary 1,2-shift, elimination, aromatization and intramolecular aldol reactions provide novel reaction pathways. In a hydrophobic environment, the thiol-capture function of the ketone moiety facilitates electrophilic attack.

The predictive value of animal data in human cancer risk assessment

Carcinogenic effects of chemicals can be investigated in animal experiments and epidemiological studies of exposed humans, mostly in the workplace. In this article epidemiologic evidence is compared with the animal data for 35 chemicals. Risk calculations are compared for 22 chemicals. The chemicals showing no or unclear carcinogenic effects in humans were more likely to show toxic side effects in the animal studies, indicating that the test concentrations were above the maximum tolerated dose. In addition, the animal experiments with these chemicals more often showed neoplastic effects on multiple sites than chemicals for which clear positive epidemiological studies are available. These findings may explain the existence of discrepancies between the outcomes of animal testing and human studies. They suggest that carcinogenic effects in multiple organs in animals could be seen as ultimate manifestations of the side effects of the testing method and that they have limited predictive value for the human situation.

Metabolism of 1,3-butadiene to butadiene monoxide in mouse and human bone marrow cells

1,3-Butadiene (BD), a gas used in the production of rubber and plastics, induces a high incidence of leukemias and lymphomas in B6C3F1 mice. Because of the potential involvement of the hematopoietic system in response to BD, we have examined metabolism of BD by B6C3F1 mouse and human bone marrow and by purified human myeloperoxidase (MPO), an enzyme rich in bone marrow. BD was metabolized to butadiene monoxide (BMO) by MPO and by mouse and human bone marrow cells. In all of these systems metabolism was stimulated by hydrogen peroxide suggesting a peroxidase-mediated process. In B6C3F1 mouse bone marrow cell lysates, hydrogen peroxide but not NADPH stimulated metabolism suggesting that cytochrome P450 was not involved in BMO formation. Metabolism of BD to BMO in hydrogen peroxide-fortified mouse bone marrow cell lysates was more than two orders of magnitude lower than in either NADPH-fortified rat or mouse hepatic microsomes. Experiments using both mouse and human bone marrow cells showed that cells from both sources could generate BMO from BD. These data show that BD can be converted to BMO in a target organ of BD carcinogenicity.

LacZ transgenic mouse models: their application in genetic toxicology

Gene mutations have been implicated in the etiology of cancer, developmental anomalies, genetic disease and aging. Many different methods for mutation detection have been developed and applied to obtain a more fundamental insight in the chain of molecular events that ultimately lead to mutations. Most of these methods, however, can only be applied to cultured cells and therefore do not allow comparative analysis of mutations in various organs and tissues in an intact organism. The main difficulty in studying mutagenesis in chromosomal DNA is to identify and isolate mutated genes with a high efficiency. Here we describe the development and application of LacZ transgenic mouse models for studying, in different organs and tissues, spontaneous or induced mutations. Such models allow study of the induction of DNA damage, repair, mutagenesis and carcinogenesis in one animal system. Accordingly, results obtained may ultimately provide greater insight into the chain of events from in vivo exposure to genotoxic agents to mutations and their ultimate physiological endpoints. In addition to their use in fundamental research, transgenic animal mutation models find a major application in the field of genetic toxicology testing, in particular with respect to organ specificity.

Molecular cloning of three sulfotransferase cDNAs from mouse liver

Sulfate conjugation plays an important role in the biotransformation of not only xenobiotics but also many endogenous substances. Sulfotransferases, the enzymes that are responsible for this process, exist as a superfamily of genes. It has long been recognized that significant species differences exist among drug and carcinogen metabolizing enzymes such as cytochrome P450. Species differences in both regulation and catalytic activities of sulfotransferases may also exist. To investigate this, we conducted cDNA cloning and cDNA expression studies of sulfotransferase in the mouse. Three sulfotransferase cDNA clones were isolated from a female B6CBA mouse liver. Two of the clones, mSTa1 and mSTa2, were highly homologous to each other. Alignment of mSTa1 and mSTa2 cDNAs' nucleotide sequences with those of other sulfotransferase cDNAs revealed the greatest sequence identity with the rat STsmp cDNA. This analysis suggests that mSTa1, mSTa2 and rSTsmp cDNAs are derived from orthologous genes belonging to the alcohol/hydroxysteroid sulfotransferase gene family. The third clone, mSTp1 showed high identity to rSTp, hSTp1, hSTp3, and rSTp1C1, suggesting that mSTp1 belongs to the phenol family.

Cell immortalization as a key, rate-limiting event in malignant transformation: approaches toward a molecular genetic analysis

Recent advances using somatic cell genetic approaches have provided a convincing body of evidence that the senescence of mammalian cells in culture is controlled by a small group of genes, one or more of which are functionally deleted in the process of immortalization. Microcell-mediated mono-chromosomal transfer methods should permit precise mapping of these genes to specific chromosomal regions. Cloning of senescence genes, using either conventional 'positional cloning' techniques or retroviral insertion mutagenesis, is now a realistic possibility. The leap in our understanding of the molecular genetic events driving the alternative cellular states of limited proliferative capacity and immortality, which such advances should precipitate, will finally permit the question of the role of cell immortalization in cancer to be addressed, and may open the door to the design of new modes of cancer therapy. In addition, the precise mechanism underlying the wide difference in transformability between human and rodent cells, which should also emerge from these investigations, is likely to make a significant contribution towards resolving the key issue of the relevance of rodent tumour induction assays in assessing the potential carcinogenicity of environmental chemicals.