NEW JERSEY - Tea, made from the dried leaves of the plant Camellia sinensis, is consumed by a large proportion of the world’s population: it’s the most popular beverage after water. In a recent review published in the journal Nature Reviews Cancer, a group of American researchers of the Center for Cancer Prevention Research in Rutgers, New Jersey, discuss the current understanding of how tea constituents might prevent cancer.

Tea extracts, especially those from green tea, have been shown to inhibit the formation and development of tumours at different organ sites. There is considerable evidence that tea polyphenols inhibit enzyme activities and signal transduction pathways, resulting in the suppression of cell proliferation and enhancement of apoptosis, as well as the inhibition of cell invasion, angiogenesis and metastasis.

Among tea polyphenols, epigallocatechin-3-gallate (EGCG), the most abundant catechin in green tea, is a potent antioxidant that holds promise for cancer prevention.

The prevention of oxidative stress, the modulation of carcinogen metabolism and the prevention of DNA damage have been suggested as possible mechanisms in the prevention of cancer.

Green tea against lung cancer

Cancer prevention by tea and tea components has been studied in many different models of carcinogenesis. For lung cancer, there is evidence from a randomised intervention study, in which supplementing of the diets of heavy smokers with 4 cups of decaffeinated green tea (73.5 mg of catechins per cup) for 4 months was found to reduce levels of urinary 8-hydroxydeoxy-2'-deoxyguanosine (8-OHdG), a biomarker of oxidative DNA damage, by 31% compared with the control group. Similarly, administration of green tea to smokers for 4 weeks reduced the number of 8-OHdG-positive cells to 50% of the pretreatment level, the researchers report.

An interesting approach in cancer prevention may be the use of polyphenols in combination with other agents. The review-authors recently demonstrated the synergistic inhibitory action of a combination of a standardised green tea polyphenol preparation and the statin atorvastatin. In the animal model as well as in human lung cancer cell lines, treatment with green or black tea for 60 weeks also inhibited lung carcinogenesis. The combination of a standardised green tea polyphenol preparation with atorvastatin synergistically inhibited lung tumorigenesis in mice and the growth of lung cancer, possibly through enhanced apoptosis, whereas the individual agents were not effective in inhibiting lung tumorigenesis.

Reviewing a total of 147 papers published up to December 2008, the authors found evidence of different cancer-preventative or inhibitory effects of tea such as the inhibition of lung tumorigenesis by high doses of oral caffeine or the inhibition of oral/digestive tract tumorigenesis by orally administered concentrates of pure EGCG. Evidence from various animal studies shows that tea polyphenols inhibit enzyme activities and signal transduction pathways, resulting in the suppression of cell proliferation and enhancement of apoptosis, as well as the inhibition of cell invasion, angiogenesis and metastasis.

“Tea preparations exert an inhibitory effect at all stages of lung carcinogenesis. However, high doses of tea polyphenols are required to produce a cancer-preventative effect, possibly owing to the relatively low bioavailability of EGCG,” the authors conclude.

Oncogenes more susceptible to epigallocatechin-3-gallate?

It remains a challenge to demonstrate the specific molecular events that are responsible for cancer prevention in animal models and humans. An interesting phenomenon that has been observed in different laboratories is the greater susceptibility of cancer cells (compared with normal and non-transformed cells) to the inhibitory effect of EGCG and other chemopreventative agents. A possible explanation for this phenomenon is the concept of oncogene addiction.

According to this concept, the rapid growth of cancer cells depends on the aberrant activity or overexpression of a few oncogenes; if EGCG blocks the activity of one or two of these oncogenes, cell growth would be severely inhibited and apoptosis would occur. Conversely, growth and survival of normal cells depends on the expression of many sets of pathways: inhibition of one or two of them by EGCG would not significantly affect cell survival. The proposed cancer prevention mechanisms involving antioxidative and pro-oxidative actions and elimination of environmental carcinogens also need to be further investigated.

“On the basis of these results, we think that tea, which is readily available and widely consumed, has a high potential for application in the prevention of human cancer.”

Reference:

Yang CS et al. Nat Rev Cancer. 2009 Jun;9(6):429-39.