CHEMICALS suspected of causing cancer usually are tested one at a time to see whether they are carcinogens. There is a problem with this approach, however. Outside the test laboratory we are exposed to many cancer-causing agents simultaneously. And some may work together to produce more cancers than would be produced by any acting alone.
The artificial sweetener saccharin may act in this way. According to a recent report from the National Academy of Sciences, the sweetener is a very weak carcinogen by itself. Although feeding saccharin to laboratory animals will cause some of them to develop cancers of the urinary bladder, large doses are needed when it is the only carcinogen given the animals. In comparison, very small doses of more powerful carcinogens can produce the same incidence of bladder tumors.
The academy report also states that saccharin promotes the growth and development of tumors caused by the action of other carcinogens when test animals are exposed first to the other carcinogen and then fed the sweetener. The report concludes: "Since humans are exposed to a variety of chemical carcinogens in their environment, the carcinogenic risk from saccharin as a promoter may be considerably greater than that indicated by the single compound studies."
The phenomenon of tumor promotion has received relatively little attention outside the community of scientists who are trying to learn just how chemicals cause cancer. Nevertheless, recent research suggests that promotion may play an important role in the development of such common human cancers as those of the bladder, colon, liver and lung. The list of suspected promoters now contains a number of substances to which humans are exposed, including saccharin; the bile acids, which are normally present in the intestines; drugs such as phenobarbital, a widely used and abused sedative for humans, and some of the chemicals in cigarette smoke.
Most of what we know about tumor promotion comes from studies of the induction of skin tumors in mice, however. The mouse research began about 40 years ago, largely as the result of a discovery by Isaac Berenblum, now at the Weizmann Institute in Rehovot, Israel. Berenblum found that the development of the skin tumors could be broken down into two steps, now called initiation and promotion.
The initiating step is exposure of the animals to a single very low dose of a carcinogen, which might be painted on the shaved skin of the mice. The amount applied is so small that the carcinogen alone does not cause a detectable number of skin tumors in the animals. (As the dose of a carcinogen gets smaller, proportionately fewer of the exposed animals will get cancer and more time will be required for tumors to develop in a significant number. If the dose is low enough, the animals may live out their normal life span and die of other causes before a detectable increase in cancer incidence occurs.)
The second step is promotion. Mice that have been exposed to the initiating carcinogen are repeatedly treated with very small amounts of the promoting chemical for several weeks or months. Only then do the mice develop skin cancers. They develop the cancers even though the promoter (a chemical designated TPA)most often used in the mouse experiments is not a carcinogen by itself.
Animals which undergo the promoter treatment without first having been exposed to the initiating carcinogen develop no more tumors than do control animals that have not been exposed to either of the agents. Thus, two chemicals, neither of which causes tumors when given alone under the conditions of the promotion experiment, interact to produce a large number of skin cancers.
For many years promotion could be studied only in mouse skin, a problem that caused many cancer researchers to wonder whether the experiments were producing any useful information about other tumors in other species. Within the past few years, however, a number of investigators have turned up evidence that several kinds of tumors could be induced by a two-step process like that described by Berenblum. Promotion may be a common phenomenon after all.
Bladder tumors in rats are among those that can be induced by a two-step process with either saccharin or another sweetener, sodium clyclamate, serving as the promoter, according to E. Marian Hicks and her colleagues at the Middlesex Hospital Medical School in London. (The Food and Drug Administration banned cyclamates as food additives a few years ago, and is trying to ban saccharin, too, a step that has been blocked by Congress for the time being.)
The investigators found a very low incidence of bladder tumors in rats given either sweetener by itself. As Hicks cautiously says, the incidence was "so low that it cannot be stated unequivocally that either sweetener is a solitary bladder carcinogen." If the animals first received a single dose of carcinogen before being exposed to saccharin or sodium cyclamate, they experienced a very high incidence of the tumors. Here again the dose of carcinogen was so low that it did not cause cancers by itself.
Experiments performed by Charles Heidelberger and S. Mondal of the University of Southern California Comprehensive Cancer Center also suggest that saccharin is a promoter. They had earlier devised a way of studying promotion in cells growing in laboratory dishes. Heidelberger and Mondal showed that they could alter these cells so that they resemble the malignant cells of tumors by a two-step treatment of initiator followed by a promoter. Saccharin behaves like a promoter in this model system. Heidelberger and Mondal say that saccharin does not exactly mimic the effects of the standard promoter TPA on the cells but the resemblance is close enough to suggest that the sweetener is a promoter.
Despite the evidence that saccharin is a promoter, attempts to find a link between its consumption by humans and an increased risk of bladder or other cancers have provided contradictory results. Some studies have found a higher incidence in saccharin users; others have not. Thus the debate on whether saccharin is a serious human health hazard will no doubt continue in Congress and the FDA.
Fat and Colon Cancer
RECENT EVIDENCE also suggests that the naturally occurring bile acids are tumor promoters, according to Banduru Reddy, John Weisburger and Ernst Wynder of the American Health Foundation. This effect of the bile acids may help to explain how high-fat diets might cause colon cancer in humans.
Over the past few years, studies of the eating habits and cancer rates of human populations have indicated that diets high in fat are correlated with an increased incidence of colon cancer. The correlation suggests that high-fat diets cause the cancer but does not prove that they do. The apparent connection between the two conditions might be simple coincidence. The existence of a plausible mechanism to explain how a high-fat diet produces colon cancer would tend to support the idea of cause and effect, however.
Bile acids are made in the liver, stored in the gall bladder and released into the intestines where they are needed for the normal absorption of fats. Reddy, Weisburger and Wynder propose that high-fat diets stimulate the release of the bile acids into the intestine. The acids (or possibly compounds to which the acids have been converted by the bacteria living in the intestines) then promote the development of colon cancers initiated by the action of carcinogens which made their way into the intestine, perhaps in the food. Experiments with rats performed by the investigators support their hypothesis. Wynder is a strong proponent of a "prudent" diet low in fat and high in fiber partly because he thinks it can help to reduce the high incidence of colon cancers in developed countries such as this one.
Several investigators, including those in the laboratories of Carl Peraino at Argonne National Laboratory and Henry Pitot of the University of Wisconsin Medical School, have shown that phenobarbital acts as a promoter of liver cancers in animals.
The big question, of course, is whether the results obtained from studies of tumor promotion in animals can be applied to decrease the human risk of cancer. Some results from the work on the mouse skin cancers in particular might encourage cancer researchers to think that this may be the case, if - and only if - promotion of human cancers proves identical to that in mouse skin.
An especially intriguing conclusion from the studies of the promotion of mouse skin cancers is that the effects of promoters are reversible, at least up to a point. If the exposure is stopped soon enough, or even if the intervals between promoter doses are long enough, the skin cancers do not appear in the mice. Thus, limiting human exposure to known promoters might help to decrease the risk of cancer. In fact, there is some evidence that this is the case.
Cigarette smoke apparently packs a double wallop when it comes to causing lung and other cancers. Although it contains a number of known carcinogens, researchers think that their concentrations are too low to account for the high incidence of cancer associated with smoking.
But cigarette smoke also contains several promoters and these may be responsible for the development of the excess cancers not explained by the carcinogen content. Some cancer researchers, including Wynder, think that the fact that promotion is reversible may help to explain why smokers who kick their habit enjoy a decrease in their risk of developing cancer.
Moreover, some chemicals have been found to prevent the effect of promoters on mouse skin and on cells grown in the laboratory. It is far too early to tell whether these or other chemicals will be used to prevent cancer in humans, but it is safe to say that researchers are going to study promotion inhibitors carefully.
Eluding the Tests
MEANWHILE, the FDA regulates true carcinogens and promoters in the same way. The agency will ban, or try to ban, food additives like saccharin, for example, if they are shown to cause cancer. It does not matter to the population at risk if the cancer-causing effect of a chemical is direct or indirect.
The only problem might be if a promoter slipped undetected through the tests normally used to screen for carcinogens. Herbert Blumenthal of the FDA's Division of Toxicology says he expects that promoters would be caught by the tests using animals because the animals can never be completely protected from all carcinogens capable of initiation. That animal tests may not necessarily uncover promoters by themselves is illustrated by a report from the National Cancer Institute on a bile acid called lithocholic acid. NCI did not find lithocholic acid to be a carcinogen in their tests on rats and mice, although it is an effective promoter, according to the American Health Foundation workers. Tests like the one devised by Heidelberger and Mondal, however, may provide an easy way of screening for promoters.
The scientists who study cancer-causing chemicals have known for a long time that one agent may enhance the carcinogenicity of another. Because they know that the human environment contains many carcinogens, some manmade and some natural, they consider such interactions to be vexing problems.
The research on tumor promotion is certainly not going to solve all the problems related to chemical carcinogenesis, but it is providing clues leading to a better understanding of how cells become cancerous and it may even provide some useful guides for decreasing the cancer risks of human populations.