By Will Dunham
WASHINGTON
(Reuters) - Researchers said on Monday they have confirmed the
existence of a natural system used by the body to defend against
the cancer-causing effects of toxic chemicals in food and the
environment.
Scientists
had long suspected that such a system existed, but researchers
at Johns Hopkins University in Baltimore and Tsukuba University
in Japan said they used tests involving genetically engineered
mice to confirm it after 20 years of research.
The system
appears to be a common one in many animals, the researchers said,
adding that they were seeking ways to use the corresponding system
in the human body to help protect people from cancer.
The body's
protective system hinges on a sharp boost in protective enzymes
-- called phase II enzymes -- that can dispose of toxic chemicals.
The enzymes effectively neutralize toxins' ability to damage DNA
and trigger cancer, the researchers said.
Scientists
already knew that natural substances in plants, such as the sulforaphane
in broccoli, as well as some synthetic chemicals, could tap into
this system to provide a protective effect.
In two studies
appearing in the Proceedings of the National Academy of Sciences
(news - web sites), the researchers said they not only demonstrated
the basic workings of the system in mice, but also found a ``switch''
that regulates it.
``We've gained
long-awaited proof of a basic mechanism that can reduce the risk
of cancer,'' Dr. Paul Talalay, a Johns Hopkins molecular pharmacologist
who participated in the research, said in a statement.
He said that
raising the levels of phase II enzymes can provide a ``highly
effective way'' to guard against cancer.
``Our precise
understanding of this system should make it fairly easy to design
drugs that can fine-tune it,'' added Johns Hopkins researcher
Thomas Kensler, a toxicologist who is overseeing early clinical
trials of one such drug in China.
``We have
evidence that we can increase the system's levels of protection
in people and are planning long-term studies that would reveal
any lowered incidence of cancer.''
The researchers
focused on strategies that cells use to control activity of the
phase II enzymes. They knocked out the ''switch'' -- a protein
called Nrf2 -- in a bid to remove the protective system in genetically
engineered mice.
The researchers
exposed both the ``switched-off'' mice and normal mice to benzpyrene,
a potent carcinogen found in cigarette smoke. They said both types
of mice developed tumors, but the ``switched-off'' mice had many
more.
In addition,
scientists gave both the normal and the ''switched-off'' mice
a drug called oltipraz, which is used for parasite infections
and has been shown to reduce cancer risk, along with the cigarette
smoke carcinogen. The normal mice had their tumor number cut by
half, but the other mice were tumor-ridden, even with the protective
drug.
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