MECHANISMS OF 7,12-DIMETHYLBENZANTHRACENE CARCINOGENESIS

Elucidation of the mechanisms of activation of chemical carcinogens
is central to understanding the process of cancer initiation by
chemicals and in designing preventive strategies. A powerful
approach to this problem is to identify carcinogen-DNA adducts.
Metabolic activation of polycyclic aromatic hydrocarbons (PAH) can
be understood in terms of two main mechanisms: one-electron
oxidation to form intermediate radical cations and monooxygenation
to produce bay-region diol epoxides. The overall objective of this
research is to demonstrate that these mechanisms of activation
occur in vitro and in vivo by identifying DNA adducts formed with
the potent carcinogen 7,12-dimethylbenz(a)anthracene (DMBA) and
several related PAH. Primary emphasis will be placed on adducts
formed by one-electron oxidation, but the analytical techniques
employed will identify adducts formed by both mechanisms. The
central hypothesis is that adducts formed by one-electron oxidation
of DMBA contain a covalent bond between one of the methyl groups
and DNA, whereas adducts formed by monooxygenation arise from
reaction of a diol epoxide that binds to DNA at C-1. This
comprehensive research approach is based on the fact that
cytochrome P-450 can catalyze activation of chemicals by one-
electron oxidation as well as monooxygenation. DNA adducts formed
by DMBA (highly carcinogenic), 1,2,3,4-tetrahydro DMBA (highly
carcinogenic despite its saturated angular benzo ring) 5-fluoro
DMBA (weakly carcinogenic) and 9,10-dimethyl-anthracene
(noncarcinogenic) will be analyzed by fast atom bombardment tandem
mass spectrometry and fluorescence line-narrowing spectrometry
(FLNS). Mechanistic studies of the PAH radical cation chemistry
will be conducted by the UNMC and UN-L groups. Synthesis of
adducts as well as biochemical and biological experiments will be
conducted by the UNMC group. Mass spectrometric analysis of adduct
structures will be conducted at UN-L, whereas fluorescence line-
narrowing spectrometric analysis of nucleoside, DNA and globin
adducts will be conducted at ISU. The studies will enable us to
assess the roles of one-electron oxidation and monooxygenation in
metabolic activation of DMBA. Furthermore, they provide a
systematic framework to develop MS and FLNS techniques for
resolving biological problems.