Study Reevaluates Role of Carcinogen-Activating Enzyme

Many drugs, pollutants and other environmental chemicals are inert until they are metabolized (acted upon by enzymes) to reactive intermediates that can bind to DNA, cause mutations, and ultimately cancer. Results of a new study emphasize the importance of studying the effects of pollutants such as  4-aminobiphenyl (ABP) in live animals. ABP was formerly used in the dye and rubber industries until it was found to cause bladder cancer in humans. Numerous previous studies––in a test tube, chemical flask or in cell culture––have shown that ABP is almost exclusively activated to reactive intermediates by an enzyme called CYP1A2; these intermediates bind to DNA, which has been associated with the development of bladder cancer. However, a new study in mice, appearing in the August 20 edition of the Journal of the National Cancer Institute, does not appear to support this model.

Yutaka Tsuneoka, MD, PhD, former postdoctoral fellow of the UC College of Medicine, and Daniel W. Nebert, MD, professor in the UC Department of Environmental Health, found that mice with the CYP1A2-producing gene “turned on” had not more (as expected) protection from ABP intermediates, but less protection (after ABP treatment) than similarly treated mice that do not have the CYP1A2-producing gene turned on. The authors conclude that either CYP1A2 is not the major enzyme that activates ABP in the intact animal or that other enzymes metabolically activate ABP if the gene for CYP1A2 is absent.

“Our knockout mouse model has turned this theory on its head,” said Dr. Nebert, “This further emphasizes the need and the importance of lab animal studies.”

Due to subtle differences in treatment (if the animal inhales or ingests a chemical, or receives it by injection) and in the target organ (whether the tissue is in direct contact with the chemical or is in indirect contact only through the bloodstream), findings in the intact animal can differ profoundly from studies performed only outside the animal. According to Dr. Nebert, textbooks for more than 10 years have taught that CYP1A2 is necessary for the formation of ABP reactive intermediates, but this fact has only been demonstrated in test tubes and cells in culture.

“We expected that a mouse having the genetic absence of CYP1A2 would be protected from ABP intermediates, compared with an ABP-treated mouse having large amounts of CYP1A2, and we found just the opposite,” said Dr. Nebert.  “We have two more exciting knockout mouse models in our lab in which our findings also contradict previous dogma that had been ‘established’ by experiments only outside the animal––therefore providing strong reasons not to eliminate animal research when studying human medical disease conditions.”

Other authors for the study include Timothy P. Dalton, PhD, assistant professor; Marian L. Miller, PhD, research associate professor; Corey D. Clay, graduate student; Howard G. Shertzer, PhD, professor; Glenn Talaska, PhD, associate professor; and Mario Medvedovic, PhD, assistant professor, all from the UC Department of Environmental Health.