The potential of genetically altered mice as animal models for carcinogen identification

Risk of human health by particulate matter as a source of air pollution--comparison with tobacco smoking

Increased air pollution, containing carcinogenic particulate matter smaller than 2.5 microm (PM(2.5)), has gained particular attention in recent years as a causative factor in the increased incidence of respiratory diseases, including lung cancer. Extensive carcinogenicity studies conducted recently under Good Laboratory Practice conditions by National Toxicology Program in the USA, Ramazzini Foundation in Italy or Contract Research Organizations on numerous chemical compounds have demonstrated the importance of considering dose levels, times and duration of exposure in the safety evaluation of carcinogenic as well as classical toxic agents. Data on exposure levels to chemical carcinogens that produce tumor development have contributed to the evaluation of human carcinogens from extrapolation of animal data. A popular held misconception is that the risk from smoking is the result of inhaling assorted particulate matter and by products from burning tobacco rather than the very low ng levels of carcinogens present in smoke. Consider the fact that a piece of toasted bread contains ng levels of the carcinogen urethane (ethyl carbamate). Yet, no one has considered toast to be a human carcinogen. Future human carcinogenic risk assessment should emphasize consideration of inhalation exposure to higher levels of benzo (a) pyrene and other possible carcinogens and particulate matter present in polluted air derived from automobile exhaust, pitch and coal tar on paved roads and asbestos, in addition to other environmental contaminant exposure via the food and drinking water.

Transgene stability and features of rasH2 mice as an animal model for short-term carcinogenicity testing

The transgenic mouse rasH2 line, in which the mouse carries the human c-Ha-ras gene under the control of its own enhancer and promoter, has been proposed as one of the alternative short-term models for carcinogenicity testing. To apply this purpose, we have produced a genetically homogeneous population as C57BL/6JJic-TgN(RASH2) (Tg-rasH2) by continuous backcrossing. In this study, we examined the transgene stability between different generations and the detailed transgene architecture of the integrated human c-Ha-ras gene. Fluorescence in situ hybridization analysis showed that the integrated human c-Ha-ras gene was stably located on chromosome 15E3 in Tg-rasH2 mice at generation number (N) 15 and 20. Southern and Northern blot analysis did not show any differences in the hybridized band pattern in each generation. Southern blot analyses showed that the Tg-rasH2 mouse contained three copies of the human c-Ha-ras gene arrayed in a head-to-tail configuration. We also determined the nucleotide sequence of the transgene in the Tg-rasH2 mouse at N20 and confirmed that the sequence of the coding region was perfectly matched with human c-Ha-ras cDNA. Cloning and sequencing of genome/transgene junctions revealed that integration of the microinjected human c-Ha-ras gene into mouse host genome resulted in a 1820-bp deletion in the rasH2 line. The deleted sequence did not have any sequence homologies with known functional genes. We assumed that either the deletion or the transgene insertion, or both, would not cause insertional mutation. In short-term carcinogenicity testing with a genetically engineered mouse model, confirmation of the transgene or modified gene stability at each generation is one of the important factors that affect the sensitivity to carcinogenic compounds in the same way as the genetic background, age and route of administration.

Induction of drug metabolism-related enzymes by methylcholanthrene and phenobarbital in transgenic mice carrying human prototype c-Ha-ras gene and their wild type littermates

Transgenic mice hemizygously carrying human c-Ha-ras proto-oncogene, Tg-rasH2 show very sensitive and facilitated carcinogenicity to various carcinogens. In this study, activities of certain enzymes related to drug metabolism and energy metabolism were measured in microsome and cytosol fractions of livers of Tg-rasH2 mice and their wild type littermates with both sexes treated with 3-methylcholanthrene (MC) and phenobarbital (PB). Aminopyrine N-demethylase activities increased significantly in livers of all mice treated with PB. MC and PB treatments induced significant increases in activities of UDP-glucuronosyltransferase and S-adenosyl homocysteinase compared to those in the non-treated groups in microsome fractions from all mice. In cytosol fractions of livers of all mice, glutathione S-transferase activity was significantly induced in the PB treated groups. There were no significant differences in activities of lactate dehydrogenase, glucose 6-phosphate dehydrogenase, pyruvate kinase and glucose 6-phosphatase related to energy metabolism in livers and kidneys among all mice. Tg-rasH2 mice showed stable activities of enzymes related to drug detoxication and energy metabolism similar to those of non-transgenic mice. These results suggest that the human c-Ha-ras transgene may not affect drug metabolism-related enzymes, and the facilitated carcinogenic response in the Tg-rasH2 mouse is not due to these enzymatic disorders.