The Antimutagenic and Anticarcinogenic Effects of Wheat Grains
Zhen
Yu
LPI Graduate Student
Wheat accounts for one third of the total grain production worldwide. There are three parts to each kernel—the endosperm, bran, and germ (see figure below). The bran and germ are separated from the starchy endosperm, which is ground to make flour during the milling process. Whole wheat has less fiber than bran, and refined wheat has the least amount of fiber. Amounts of other phytochemicals also vary. It is not yet clear which constituents of whole wheat may offer significant protection against cancer.
Heterocyclic
amines (HCAs) are a group of carcinogens and mutagens formed in cooked
meat and fish. Some HCAs, such as 2-amino-3,4-dimethyl-imidazo[4,5-f]quinoline
(MeIQ), 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx),
and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), have
been judged as possible human carcinogens, and 2-amino-3-methylimidazo[4,5-f]quinoline
(IQ) as a probable carcinogen, by the International Agency for Research
on Cancer. HCAs require metabolic activation to damage DNA. They can be
activated by phase I enzymes in the liver and detoxified by phase II enzymes.
HCAs that are not detoxified may react with DNA to cause mutations and
cancer. Anti-carcinogens may act against HCAs by inhibiting phase I enzymes
and enhancing phase II enzymes, resulting in the detoxification of HCAs,
or by binding directly to the carcinogen to reduce its ability to damage
DNA.
In the male rat, chronic administration of IQ or PhIP induces multiple tumors, including adenocarcinomas of the colon, and short-term treatment leads to the formation of aberrant crypt foci (ACF) in the colon, which are early biomarkers for colon cancer (see figure below). In studies directed by Dr. Roderick Dashwood of LPI, the effects of unrefined whole wheat, refined wheat, and wheat bran on the mutagenicity of HCA were assessed using the Salmonella assay, a bacterial mutagenicity test that is used as a screen for potentia carcinogens. In this assay, also known as the Ames Test, Salmonella bacteria are exposed to suspected mutagens to evaluate mutagenicity. Putative anti-mutagens may also be incorporated into the protocol. In tumor studies, male rats were treated with IQ and fed supplementary whole wheat or refined wheat.
The
pH, a measure of acidity or alkalinity, changes along the length of the
gastrointestinal intestinal (GI) tract. The stomach is very acidic with
a pH of about 1.8. The ascending colon is less acidic with a pH of 5.5,
and the small intestine has a neutral pH of 7.0. We tested the mutagenicity
of MeIQx at these different pHs using the Salmonella assay. At
the stomach pH, there was no significant inhibition of MeIQx mutagenicity
by refined wheat, moderate inhibition by unrefined whole wheat, and close
to maximal inhibition by wheat bran. Inhibition by refined wheat and unrefined
whole wheat improved as the pH increased to that of the ascending colon
and small intestine, but the inhibitory effect of bran was unaffected
by pH. We conducted another experiment to mimic conditions in the body
by treating wheat grains with the digestive enzymes pepsin and pancreatin
found in the stomach and then incubating with MeIQx. The results indicated
that enzymatic digestion may attenuate the antimutagenic effects of cereals
against MeIQx.
We wanted to find out if the antimutagenic effect of cereals is due to their binding to mutagens, so different amounts of wheat grains were pre-incubated with MeIQx, PhIP, or IQ in solution. Aqueous fractions of all of the grain samples significantly inhibited the mutagenicity of the HCAs in the Salmonella assay, with greater inhibition correlated with increasing grain concentration. Bran was more effective than whole wheat in these experiments. Next, we incubated grains without mutagens and then tested the aqueous fraction against the mutagens using the Salmonella assay, finding significant inhibition by refined and unrefined wheat, but not by bran. These results indicated that the antimutagenic effect of refined and unrefined whole wheat does not depend on the possible binding of mutagens to cereal components.
With these encouraging in vitro data, we next examined the in vivo relationship between dietary wheat and the formation of HCA-induced ACF in rats. We found no differences in ACF incidence in rats fed unrefined whole wheat or a refined wheat diet before or after exposure to the carcinogen, nor did we find any significant differences in the average number of ACF per colon or in the average number of aberrant crypts per colon. However, rats given unrefined whole wheat after the carcinogen exposure had fewer of the smaller sized ACF than rats fed the same diet before and during carcinogen treatment. These results suggested weak effects, at best, by whole wheat and refined wheat diets against mutagen-induced ACF in vivo, although we can’t rule out the possibility of a larger effect with prolonged feeding.
In our rat study,
the protocol did not focus on direct interactions between the mutagen
and cereal. Instead, we tested the hypothesis that phytochemicals released
from whole wheat and refined wheat in vivo might inhibit mutagens,
rather than binding to them in the GI tract. Because we observed only
marginal effects of wheat on IQ-induced ACF and on the activities of phase
II enzymes in the animal study, it is possible that co-administration
of cereal and mutagen might have been more effective.
Another caveat concerns the relative levels of the cereals and mutagens
in the Salmonella assay and in the rat study and how these might
correspond with human intake. In animal studies, the grains were added
to other components of the diet, and based on our calculations, the corresponding
weight ratios of grain to mutagen would be approximately 67,000,000:1
in human diets, 10,000:1 in the Salmonella mutagenicity experiments,
and only 350:1 in the rat study. If we assume that the weight ratio of
grain to mutagen is an important determinant of the degree of inhibition
of the mutagen, it is perhaps not surprising that only marginal changes
were detected in the rat study. Therefore, it may be warranted in future
studies to isolate and purify individual antimutagens released during
aqueous incubation of the cereal grains and then test these compounds
in rats using doses of HCA mutagens relevant to human exposure.
Last updated May, 2003