PRINCIPLES OF TOXICOLOGY - Biology East Borneo

298 CHEMICAL CARCINOGENESISIn rats, hyperplastic and neoplastic changes in the forestomach may result from the chronic administrationof compounds like butylated. Once again, however, the relevance to humans of such responsesis not known.Physiologic DifferencesMale rats produce a protein known as α-2-microglobulin, which, in combination with certain chemicalsor their metabolites, causes a repeated cell injury response in the proximal tubules of the kidney.However, significant levels of α-2-microglobulin are not found in female rats, mice, or humans. Thus,the mechanism believed responsible for the repeated cell injury and tumors formation observed in malerats does not exist in these species. The male rat kidney tumors observed after chronic gasolineexposure, or exposure to certain aliphatic compounds, such as d-limonene, are notable examples ofthis phenomena. The scientific community has concluded that the positive male rat data for suchchemicals is not relevant for predicting human cancer risk.Cellular and Biochemical DifferencesThe B6C3F 1mouse routinely used in cancer bioassays has a genetically programmed high backgroundincidence of hepatocellular cancer. Approximately 20–30 percent of untreated animals develop thistype of cancer. The B6C3F 1mouse is a genetic cross between the C3H mouse, which has almost a 60percent background rate of liver cancer, and a C57BL mouse, which has a very low incidence rate ofliver. Because the B6C3F 1mouse was bred to exhibit a genetic predisposition for developing livercancer, tests using this animal model have subsequently identified a number of chemicals that are onlyliver carcinogens in this mouse strain and not the rat. In turn, the relevance of the liver tumors whichare so commonly induced in this mouse are frequently questioned when extrapolated to humans,especially in light of the relatively low incidence with which human hepatocellular cancer occurs (3–5cases per 100,000) in the United States.The molecular mechanism for the high background cancer incidence in the B6C3F 1mouse appearsto be related to its propensity for oncogene activation in the liver. For example, the DNA of the B6C3F 1mouse H-ras oncogene is hypomethylated, or deficient in methylation. Methylation of DNA serves toblock transcription of a gene. And since the mouse H-ras oncogene is not adequately methylated (i.e.,not “blocked”), it may be inappropriately expressed more easily, thus providing a mechanisticfoundation for the higher background incidence of liver tumors in this mouse strain. Further, certaintypes of hepatotoxicity may exacerbate the hypomethylation of the H-ras gene in this sensitive species,but have no significant effect on the gene methylation rates in less sensitive species. Thus, the relevanceto humans of liver tumor development in this test species, or any other animal species which has apropensity for the spontaneous development of the tumor, is questionable.To summarize, the use of mice and rats is generally a compromise aimed at decreased costs. Whileprimates or dogs might better represent the human response to some chemicals, they cannot be usedroutinely because of the additional costs incurred and other reasons. In general, the use of rodents asa surrogate animal model for humans might be criticized because rodents typically have a faster rateof metabolism than do humans. So, at high doses the metabolic pattern and percentage of compoundultimately metabolized may be significantly different than that of humans. If the active form of thecarcinogen is a metabolite, then the animal surrogate may be more sensitive to the chemical becauseit generates more of the metabolite per unit of dose. Alternatively, the problem of false negatives alsoapplies in that the selection of an insensitive species may yield a conclusion of noncarcinogenicitywhereas further testing would uncover the actual tumorigenic activity. Because significant speciesdifferences exist in key aspects of all areas relevant to carcinogenesis (metabolism, DNA repair, etc.),and as these differences are the rule rather than the exception, extrapolating the response in any speciesto humans without good mechanistic data should be done with caution. In addition, developingmechanistic data that will allow comparisons to be made between humans and both a responsive and