| ABSTRACT: |
|
In vitro short-term tests are needed to determine the efficacy and
mechanism(s) of action of cancer chemopreventive agents. To explore the
usefulness of a microsomal system, benzo[a]pyrene (BP) (50 uM) was
activated with aroclor 1254-induced rat liver microsomes in the presence
of DNA (300 ug/ml), with and without chemopreventive agents (150 uM).
32P-postlabeling analysis of the purified DNA produced two major adducts:
one derived from BPDE, the other from 9-OH-BP. Intervention with curcumin
reduced BPDE adduct formation by approx 50% but little or no effect was
observed for the 9-OH-BP adduct. In contrast, oltipraz reduced BPDE and
9-OH-BP adduct formation by 75% and 25%, respectively, and
N-acetyl-cysteine (NAC) reduced both adducts by 25%, while ellagic acid,
which is known to conjugate with BPDE, resulted in greater than 80%
reduction. The preferential reduction of BPDE over the 9-OH-BP adduct
suggests selective inhibition of metabolic pathways. Further, oltipraz,
curcumin and ellagic acid all resulted in substantially (greater than 70%)
reduced DNA binding of 7,12-dimethylbenz[a]anthracene (DMBA); NAC,
however, had no effect. Possible mechanisms for the observed adduct
reduction include direct interaction of the chemopreventive agent with the
carcinogen or its metabolite, inhibition of phase I enzymic activity, and
potential 'shielding' of the DNA binding sites by the test agent. These
data suggest that the microsomal system may prove useful as a screening
method for cancer chemopreventive agents and to define their role in the
preinitiation phase of carcinogenesis. |