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May 20, 2005

While On Trail Of Dioxin, Scientists Pinpoint Cancer Target Of Green Tea

Topics: Medicine

It seems that we are begining to have more clarity on the mechanisms by which green tea provides protective and even therapeutic effects in cancer.

According to  scientists at the University of Rochester Medical Center, green tea appears to protect against cancer by affecting a "promiscuous" protein that pharmaceutical experts are already targeting in an effort to develop a new drug to stop the disease. The research supports current thought concerning the health benefits of green tea, with solid scientific evidence.

While on the trail of dioxin's harmful effects and thinking that epigallocatechingallate(EGCG) in green tea(the main constituent of green tea extract) bound to an aryl hydrocarbon(AH) receptor that frequently plays a role in turning on harmful genes; they instead found that EGCG binds to HSP90(great image), a protein that helps other proteins stay stable, serving the same role as a tail on a kite.  Aryl hydrocarbon receptor is a ligand-activated transcription factor known to mediate the toxic effects of numerous environmental contaminants, including the polycyclic aromatic hydrocarbons. When the two bind, HSP90 no longer turns on the AH receptor, stopping the cascade of events that would lead to the activation of genes  that can result in carcinogenisis.

(...)  Competitive binding assays strongly suggest that EGCG does not bind to the AhR ligand binding site, indicating this compound functions through a mechanism unlike that of typical AhR antagonists. Affinity chromatography experiments implicate an indirect mechanism of action involving direct binding of EGCG to the AhR chaperone protein, hsp90. This induces an AhR conformation capable of nuclear localization but incapable of binding DNA. These altered signaling events correlate with the formation of a complex with sedimentation characteristics different from those of the latent or ligand-activated AhR. These data implicate a model in which EGCG inhibits release of hsp90 from the AhR, stabilizing the complex in an intermediary state associated with XAP2.

As the researchers point out in their article, this is the first time EGCG has been demonstrated to directly bind hsp90 and the first indication that GT may exert its chemopreventive effects through an interaction with the common chaperone hsp90. Pharmaceutical companies are currently investigating ways to block HSP90, which is commonly known as a promiscuous chaperone protein because it binds to many different cells and receptors in the body. It turns out that investigators are trying to duplicate what green tea does naturally, and can learn from green tea, which might modulate HSP-90 in a way that researchers haven't seen before.

Hsp90(90 kD heat shock protein) is unique among molecular chaperones. The majority of its known substrates are signal transduction proteins, and recent work indicates that it uses a novel protein-folding strategy. In the eukaryotic cytosol, Hsp90 is termed variously Hsp90{alpha} and ß in humans (corresponding to a major and minor isoform).

While some will say that the latest results make the idea of harnessing green tea's protective power more feasible which could lead to extending the health benefits first discovered in green tea to people who never touch the beverage - that is not necessarily the case. While in some or many cases this may be true, the real problem is not knowing what got thrown out in the bath water!

Posted by Hyscience at May 20, 2005 9:39 AM



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