Objectives:

Limitations in our understanding of comparative metabolism contribute to uncertainty in extrapolating testing results from rodents to potential risks in humans. The examination of between-species differences in the metabolic activation or detoxification of xenobiotics is potentially useful for determining differential species susceptibility to reproductive toxicants. Therefore, the goal of this project is to improve reproductive risk assessment by increasing understanding of how the male reproductive system processes xenobiotics. This will increase our knowledge of basic comparative reproductive metabolism and thereby improve extrapolation of risk to humans. The specific goals are to:

1. the specific testicular enzyme systems in rats, rhesus monkeys, and humans that are responsible for metabolic activation of toxicants within the reproductive tract;
2. express selected testicular metabolizing enzyme isozymes in a human lymphoblastoid cell line, yielding high levels of expression of the individual proteins so that metabolism can be more readily investigated;
3. study chemical and endocrine influences on the expression and regulation of specific testicular metabolizing enzymes.

Experimental Approach:

We plan to determine which enzyme/isozyme systems are responsible for metabolic activation of toxicants in the testes of rat, monkey, and human. Based on the results of comparative whole tissue homogenate studies, we will express selected testicular metabolizing enzyme isozymes in the AHH-1 cell line. This will yield high levels of expression of the individual protein so that metabolism may be more readily investigated. It will also provide a model for examining xenobiotic and steroid metabolism by individual enzymes in the absence of other, potentially confounding pathways so that insights may be gained into the role of individual enzyme pathways in the overall toxicant-induced outcome. Predictions of human susceptibility to a toxicant based on the presence or absence of an activating/detoxifying pathway can then be made. Finally, we will use cultured Leydig cells to examine chemical and endocrine influences on the expression and regulation of specific testicular metabolizing enzymes in these isolated steroidogenic cells. By inducing or inhibiting particular enzymes, by exposing the cells to different hormonal environments, and by testing cells from strains expressing different spectra of enzymes, we will determine the relative importance of specific isoforms in xenobiotic activation and/or detoxification.

Expected Results:

The results of these studies will provide insights into the mechanisms of metabolic activation and detoxification in the male reproductive tract and will allow comparisons between rodent, primate and human reproductive tract metabolism. This will improve our understanding to the mechanisms of differential susceptibility to reproductive toxicants, and will allow more exact extrapolation of rodent toxicity data to potential human susceptibility, thereby reducing the uncertainty in extrapolating reproductive toxicity studies to humans.