The initiation of the cancer process is associated with mutations. Analysis of
environmental exposure to chemical or physical agents causing these genetic alterations
is of great importance in order to develop strategies for avoiding or reducing cancer risk
in humans. The causality between mutagenesis and carcinogenesis also prompts the
concept that the modifying effect on mutagenesis by a compound would be predictive of
the cancer preventive potential of that compound. The Big Blue© transgenic assay, using
the E. coli lacI gene as the mutational target provides an opportunity to evaluate
mutagenesis and its modulation m vivo. This model system was used to study the tissue-specific
effect of the potential chemopreventive agent conjugated linoleic acid (CLA), on
the mutagenicity of the suspected human carcinogen, 2-amino-1 -methyl-6-
phenylimidazo[4,5-b]pyridine (PhIP). PhIP and CLA were selected for study since both
compounds are consumed by humans on a daily basis, and are suspected to be related to
the human risk of colon, breast, and prostate cancers.
The mutagenicity of PhIP in Big Blue© rats was shown to be tissue-, sex-, and
dose-dependent. PhIP was found to be a potent mutagen in the colon, followed by the
cecum, prostate, and kidney. Compared with the background mutational spectra, the
PhlP-induced spectra were characterized by an elevated proportion o f-1 frameshifts,
consisting mainly of deletions of single G:C base pair. However, the induced spectra
varied among tissues. A sex-dependent induction of mutation by PhIP was observed in
the kidney such that the PhlP-induced mutation frequency was twice as high in male rats
as in female rats; the biological significance of this difference is not clear. In contrast,
although PhIP has been shown to induce colon tumors preferentially in male rats, and
only rarely in female rats, no difference in mutational response was detected between the
colons of male and female rats treated with PhIP.
Experiments were performed to examine the in vivo effect of CLA on
mutagenesis. Similar to what is seen for the mutagenicity of PhIP, the modification by
CLA depends on tissue, sex, and dose of administration. CLA showed a modest
protection against PhlP-induced mutagenesis in the distal part of the colon, in the
prostate, and in the kidney of female rats. However, significant changes in the overall
PhlP-induced mutation spectrum were seen only in the prostate. The antimutagenic
effect of CLA may be directly responsible for its cancer prevention «^lability, since
PhlP-induced aberrant crypt foci in the colon of male rats were completely inhibited by
CLA However, CLA was not totally innocuous. When supplemented at 0.5%, CLA
acted as a comutagen of PhIP, increasing the PhlP-induced MF in the cecum, although
this effect was not observed when CLA was supplemented at 1%. The differences in
effect may be related to the antioxidant or pro-oxidant activities of CLA isomers under
experimental conditions.
Due to the artificial nature of the lambda/LIZ lacI transgene and the possible
absence of DNA repair in this transgene, the suitability of the Big Blue© transgenic assay
as a mutational test system has been questioned. We examined the repair of UV- and
benzo(α)pyrene diol epoxide-induced DNA damage in this non-transcribed lambda
construct of the Big Blue© rat-2 transgenic cell line and demonstrated that DNA damage
is indeed repaired in this transgenic construct. Lastly, since CLA altered the mutational
spectra in the prostate in a way consistent with an effect of mismatch repair, the
possibility of an effect of CLA on mismatch repair was explored in bacteria. Although
CLA was found to increase mutant frequency in a mismatch repair proficient E. coli strain, but not in deficient strains, the mechanism by which CLA operates remains
unclear.
Altogether, the data demonstrate the mutagenicity of PhIP and its modulation by
CLA as a function of tissue, sex, and dose of administration, and support the application
of the Big Blue© transgenic assay as a screening tool for mutagens and chemopreventive
agent / Graduate
| Identifer | oai:union.ndltd.org:uvic.ca/oai:dspace.library.uvic.ca:1828/10175 |
| Date | 23 October 2018 |
| Creators | Yang, Haiyan |
| Contributors | Glickman, Barry W., de Boer, Johan G. |
| Source Sets | University of Victoria |
| Language | English, English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
| Rights | Available to the World Wide Web |
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