The primary aim of the proposed research is to determine the extent and mechanism(s) whereby naturally occurring coumarin derivatives modulate polycyclic aromatic hydrocarbon (PAH) skin carcinogenesis. Humans are continually exposed to a wide variety of chemicals, including: complete carcinogens, initiators, promoters, cocarcinogens, and anticarcinogens. Current evidence indicates that dietary ingestion of a variety of substances may either inhibit or enhance carcinogenesis in man. Coumarin substances represent one of the largest naturally occurring classes of compounds, ingested by man, yet to be fully explored for their potential effects (anticarcinogenic or cocarcinogenic) on chemical carcinogenesis. We have recently found that several natural, as well as novel synthetic coumarins, have the potential to be effective inhibitors of mouse skin carcinogenesis by PAHs. Coumarin derivatives may act as anticarcinogens by modulating both Phase I and Phase II enzymes involved in carcinogen metabolism. Therefore, we will test the hypothesis that the coumarin derivatives inhibit PAH mouse skin tumor initiation and carcinogenesis through alterations in metabolic pathways responsible for the activation and/or detoxification of specific hydrocarbons. The overall approach outlined in this proposal will allow us to evaluate whether coumarins, especially those consumed in the diet of man, increase or decrease the risk of carcinogenesis in a specific animal model system. Such information will be useful in defining whether any naturally occurring coumarins have potential chemopreventive properties or should be considered a risk factor in chemical carcinogenesis. In addition, the proposed studies will lead to a better understanding of the process of chemical carcinogenesis in a specific target tissue, mouse skin.
The specific aims are: (1) to determine the ability of naturally occurring coumarins to inhibit the formation of covalent DNA-adducts from B[a]P and DMBA in mouse epidermis in vivo; 2) to determine the dose-response, time course, and sequence of exposure relationships for the inhibitory effects of selected natural coumarins on complete carcinogenesis by B[a]P and DMBA and to determine whether the inhibitory effect is at the level of initiation, promotion, or both; 3) to examine in detail the overall mechanism whereby the most active coumarins inhibit carcinogenesis by PAH as follows: i) to determine the effects of selected coumarin on the overall metabolism of model PAH (i.e., oxidative and non-oxidative) using mouse epidermal cells in culture and/or mouse epidermis in vivo; ii) to determine the effects of selected coumarins on the activities and/or expression of specific cytochrome P-450 isozymes in mouse epidermis; iii) to determine whether inhibitory analogs inactivate cytochrome P-450s; and iv) to determine, if warranted, the effects of coumarins on the levels and expression of epidermal uridine-5'- diphosphoglucuronyltransferase (UDPGT), glutathione-S-transferase (GST), epoxide hydrase (EH), and/or sulfotransferase (ST); and 4) to determine the ability of selected natural coumarins to modulate hepatic xenobiotic and carcinogen metabolizing enzymes following oral administration.
