Researchers find green tea cancer prevention mechanism
Researchers at the University of Rochester Medical Center have discovered an anticancer mechanism for epigallocatechin gallate (EGCG), a flavonoid in green tea found to be associated with many of its benefits. Although green tea flavonoids appear to be protective against cancer, their mechanism of action had not been completely defined.
The report, which was published in the April 5 2005 issue of the journal Biochemistry (http://pubs.acs.org/journals/bichaw/index.html), was authored by Christine Palermo of the University of Rochester’s School of Medicine and Dentistry, Claire Westlake and Thomas A. Gasiewicz, PhD, who is the director of Rochester's Environmental Health Science Center and is an authority on dioxin, a well known carcinogen.
Dr Gasiewicz’s earlier work had found that EGCG blocks the aryl hydrocarbon (AH) receptor, which is affected by dioxin and other chemicals such as cigarette smoke, and can activate harmful genes. In the current research, his team found that EGCG exerts its cancer protecting ability not by binding to this receptor, but to a protein called HSP90. HSP90 is known as a promiscuous chaperone protein because it binds to a number of different cells and receptors, which helps maintain their stability. Research has shown that cancerous cells have higher levels of HSP90 than healthy cells, and that levels of proteins that encourage cancer cell growth are reduced when HSP90 is blocked.
When EGCG binds to HSP90, HSP90 no longer activates the AH receptor and the events leading to the undesirable gene activation are prevented. Dr Gasiewicz noted, “We initially hypothesized that EGCG would work in the same way as other AH antagonists, by binding directly to it. We were completely surprised that this isn't the case."
"It's important to find out the source of green tea's protective effects," Dr Gasiewicz stated. "What is exciting here is that a completely new mechanism has been found that very well could be responsible for its protective effects, and that could help us find a compound that is much more potent."
The odds ratio of stomach cancer decreased to 0.69 with a high intake of green tea (7 cups or more a day) (Inoue et al. 1998). Another study conducted in Yangzhong (a region in China with a high incidence of chronic gastritis and gastric cancer) showed the amount and duration of green tea consumption governed the rate of stomach cancer. Frequent long-term green tea drinkers had approximately 50% less risk of developing gastric cancer compared to individuals consuming little or no tea (Setiawan et al. 2001). Green tea reduces the damaging effects of nitrites in the acidic environment of the stomach with greater efficiency than vitamin C.
The growth of non-Hodgkin's lymphoma cells, transplanted in mice, was reduced by 50% when green tea was a part of the animal's diet. Cyclophosphamide, a chemotherapeutic drug, administered at the maximum tolerable dose, was unable to replicate similar benefits (Bertolini et al. 2000). Part of green tea's anticancer profile includes an antimutagenic factor that helps DNA replicate accurately (Uhlenbruck et al. 1998).
Cigarette smokers who drink green tea have a 45% lower risk of lung cancer compared to non-tea drinkers. Even though Japan has one of the highest numbers of smokers in the world, they have one of the lowest rates of lung cancer of any developed nation, a protection thought to be delivered by green tea.
