By David Leff
Hamlet cried out against "the slings and arrows of outrageous fortune," and thought about taking arms against that "sea of troubles."
Humans, and mice, take arms against the slings — organic pathogenic proteins — by deploying their immune systems' antibodies and killer cells. Against the arrows — typically, inorganic, environmental DNA-damaging substances — their genes marshal chemical weapons of cell detoxification.
In that ocean of troubles, the most predatory sharks are the cancers. Counter-attacking their tumors can require either anti-"sling" immune defenses or "arrow"-deflecting chemical detoxicants.
"Those potentially deleterious chemical agents can be produced as part of our natural environment," said molecular pharmacologist C. Roland Wolf, at Scotland's University of Dundee. "We could be exposed to them," he pointed out, "through our industrial environment, or through personal habits — such as cigarette smoking.
"A wide spectrum of genes have evolved," Wolf went on, "to protect us from our environment. Without their cytoprotective proteins, we would not survive." One such is the gene encoding the enzyme glutathione transferase.
Among the widely used industrial carcinogens is a polycyclic aromatic hydrocarbon called 7,12-dimethylbenz anthracene — DMBA for short. Besides being a cancer-causing ingredient of cigarette smoke, DMBA figures, among other things, in textile dyes, explosives, detergent odor inhibitors, and military smoke screens.
DMBA is also a boon to cancer research. "It's very commonly used as a model carcinogen," Wolf observed. "Its carcinogenic metabolites are the same as those found in cigarette smoke."
GST Elevated In Many Human Tumors
He noted that expression of DMBA's detoxicant, glutathione S-transferase [GST], "is elevated in many human tumors, including lung, colon, ovary, testis, bladder, oral and kidney." One exception, as yet not fully explained, is the enzyme's absence from prostate cancer tumors.
"Like most cancer-causing chemicals that we ingest," Wolf said, "DMBA is not of itself mutagenic. It must first be converted in the body to a form that will react chemically with DNA. Say you inhale this chemical in cigarette smoke, which gets converted to what's called an ultimate carcinogenic form. This can either interact with DNA, or be detoxified by conjugation with glutathione, a small cellular peptide. And that reaction," he continued, "is catalyzed — speeded up — by glutathione transferase."
Wolf is senior author of a research report in the current Proceedings of the National Academy of Sciences, dated April 28, 1998. Its title: "Increased skin tumorigenesis in mice lacking pi-class glutathione S-transferases [GSTpi]."
He and his co-authors created a colony of knockout mice lacking a gene that expresses one class, pi, of the cytoprotective GST enzyme.
"It turns out," Wolf told BioWorld Today, "that whereas in most species, including humans, there is only one GSTpi gene, in mice there are two. So, the targeting vector that we made was very simple, in that it just deleted one gene completely, and half of the other, which was completely nonfunctional."
The Scottish researchers then demonstrated the inability of these "have-not" rodents to pull DMBA's carcinogenic teeth.
On the mice's shaved backs, researchers daubed a single application of DMBA solution, followed by repeated cutaneous doses of a tumor-promoting agent, laid on twice per week for 20 weeks.
By 10 weeks, the co-authors could already see multiple papillomas emerging over the treated skin, but none on wild-type control animals. Of these papillomas — precancerous benign growths — 10 to 15 percent become carcinomas, Wolf said.
At week 20, 18 GSTpi-minus mice scored a total of 179 papillomas, while 19 controls had generated only 55.
"So, what we've done," Wolf summed up, "was to take one of these particular cytoprotective genes, delete it from the mouse genome, and show that that can have a very significant effect on the mouse's sensitivity to cancer-causing agents of the type found in cigarette smoke."
He pointed out that this particular gene is "highly conserved between mice and men. We know it is expressed as one of the major protective enzymes found in human lung, so we believe therefore that the level of GST enzyme in the lung will be an important determinant on deciding on the carcinogenic effects of cigarette-smoke components."
Wolf added a demurrer. "That is an extrapolation; it is not yet proven," he said.
"In our mouse skin experiments," he said, "we used the same types of chemical agents that cause lung cancer. The fact that by losing the gene the mice very profoundly increased their sensitivity to those cancer-causing agents means — in my view it's not a very big leap to make — that if you have this protein in your lungs it's going to make a difference in your susceptibility to lung cancer.
"The other part of this work that we and others have been involved with," he went on, "is that this particular gene is polymorphic in the human population. There are variant forms of it, which are known to have different activities toward cigarette-smoke carcinogens. So, obviously, what's important now is to establish whether susceptibility to smoke-induced cancer segregates as one form or another. You may be susceptible, in terms of detoxification, if you inherit a low activity variant."
A Drug To Make Smoking Safe? No Way!
Wolf and his colleagues are very interested in preventing cancer, specifically lung cancer. "It appears now," he said, "that you can give tamoxifen to women to reduce their incidence of breast cancer. What we've actually shown is that maybe just the modulation of one single gene may be sufficient to protect us from lung cancer."
Wolf added "a very strong message that I want to get across. This does not mean that people should not give up smoking, thinking we may find ways of making it safe to smoke. The message is: that's not the case.
"The main area we're interested in now is whether we can find drugs that will up-regulate the cytoprotective level of this gene. We've made a detailed analysis of its promoter, and how chemical agents may activate its elements. But we've got a few years to go on that." *