Coming - dietary aids to prevent cancer?

Coming--dietary aids to prevent cancer?

Many studies have shown that a diethigh in vegetables may offer some protection against cancer (SN: 6/26/82, p.422). Although there are a number of potential anticancer agents in vegetables--among them antioxidants like beta-carotene (SN: 10/1/83, p.217)--a growing body of evidence now suggests that the most potent protective effect may come from protease inhibitors. New studies in cultured cells and rodents indicate that these compounds, found in certain plants, may hold enormous promise as a food additive or supplement to prevent the development of human cancer.

At the Second International Conferenceon Anticarcinogenesis and Radiation Protection, held earlier this month in Gaithersburg, Md., Ann Kennedy of the Harvard School of Public Health's department of cancer biology in Boston reported on an investigation of these compounds. She and her colleagues found that certain of these plant compounds may be capable of neutralizing the effects of a wide range of carcinogens, from radiation and steroid hormones to potent components of diesel exhaust.

Found in plants' reproductive parts--including beans, rice and potatoes-- protease inhibitors are believed to provide these parts with natural protection against insect predation. But because they also block the activity of an enzyme responsible for the digestive breakdown of proteins, they gained the reputation of being antinutritious. Walter Troll of New York University Medical Center, a pioneer in protease-inhibitor cancer studies, notes with irony that "the Department of Agriculture has spent a lot of time removing protease inhibitors--from soybeans, for example--because they thought it [the removal] would make young children grow better.'

Carcinogenesis is believed to be amulti-stage process. It's initiated with exposure to a carcinogen, which triggers long-lived changes in a cell. The process is advanced when the cell is subsequently exposed to a "promoting' agent--something that may or may not be carcinogenic by itself. Promoting agents that have been established in animals or in the lab, according to the researchers, include saccharine, dioxin and constituents of cigarette smoke. It is believed that cells that have been both initiated and promoted may at some future point, during cell division, undergo a spontaneous transformation to a cancer.

What Kennedy and her co-workershave found is that even brief exposure of initiated and/or promoted cells to minute quantities of certain protease inhibitors --such as the Bowman-Birk inhibitor derived from soybeans--not only prevented the transformation of those cells into cancers, but also "reprogrammed' their precancerous change back to the pre-initiation state. And the cells weren't fussy about the timing of treatment; anytime prior to cancer transformation blocked carcinogenesis. The only real limit to the effect appears to be the dose of the initiator/carcinogen. If it is too high, the protease inhibitor may reduce-- not block altogether--tumor development.

Kennedy says some cancer researchershave labeled her findings "heresy,' on the assumption that changes during cancer initiation were irreversible. Her research now suggests that both initiating and promoting changes are indeed reversible with protease inhibitors.

To understand how these compoundswork, Kennedy and Troll are focusing on protease inhibitors' recently identified ability to inhibit the action of oncogenes. It is generally assumed that specific oncogenes must be activated for cancer to develop, Kennedy says.