Recent work published by Zhao et al in nature communications lends insight into a potential mechanism for the anti-cancer activity of green tea.
The health benefits of drinking tea, particularly green tea, has been the subject of intense investigation. A large number of epidemiologic studies have examined whether tea consumption can reduce cancer risk, with a focus on a category of polyphenols called catechins, the most active and abundant of which in green tea is epigallocatechin-3-gallate (EGCG). According to the National Cancer Institute, EGCG and other polyphenols found in green tea have multiple biological activities including being potent antioxidants, inhibiting tumor cell proliferation, inducing tumor cell apoptosis, inhibiting angiogenesis, and activating detoxifying enzymes.
In in vitro studies, EGCG has been shown to induce apoptosis in cancer cell lines. The transcription factor p53 plays a role in this activity of EGCG. P53 is an important tumor suppressor, promoting cell cycle arrest or apoptosis when a cell is stressed. Usually, cellular p53 levels are low, with p53 being ubiquitylated which targets the protein for degradation. The ubiquitylation is carried out by the protein MDM2, an E3 ligase. When cells are stressed, ubiquitylation is suppressed, p53 is not degraded, and p53 levels increase. The p53 protein then promotes apoptosis through interactions with other proteins and by inducing gene expression changes. Earlier studies have shown that EGCG stabilizes p53 and may interfere with the interaction of p53 and MDM2.
In their current study, Zhao et al demonstrate that EGCG binds to the N-terminal domain of p53. They also show that EGCG disrupts the interaction of p53 with MDM2 and inhibits ubiquitylation of p53 by MDM2. In the publication, ubiquitylation assays were carried out using fluorescein-labeled p53 and varying amounts of EGCG. After incubation with MDM2 and ubiquitin, the amount of free and ubiquitinated p53 was detected on SDS-PAGE gels using the fluorescein channel of a Sapphire Biomolecular Imager. The amount of ubiquitin was quantified and concentration-dependent inhibition of ubiquitylation by EGCG was analyzed.
The authors propose that the EGCG-induced reduction in ubiquitylation could stabilize p53 protein, making more p53 available and contributing to its anti-tumor activity. Additionally, the work suggests the N-terminal domain of p53 presents an attractive target for anti-cancer drug design.
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