Tea is one of the most consumed beverages in the world. Green tea, black tea, and oolong tea are made from the same plant Camellia sinensis (L.) O. Kuntze. Among them, green tea has been the most extensively studied for beneficial effects on diseases including cancer, obesity, diabetes, and inflammatory and neurodegenerative diseases. Several human observational and intervention studies have found beneficial effects of tea consumption on neurodegenerative impairment, such as cognitive dysfunction and memory loss. These studies supported the basis of tea’s preventive effects of Parkinson’s disease, but few studies have revealed such effects on Alzheimer’s disease. In contrast, several human studies have not reported these favorable effects with regard to tea. This discrepancy may be due to incomplete adjustment of confounding factors, including the method of quantifying consumption, beverage temperature, cigarette smoking, alcohol consumption, and differences in genetic and environmental factors, such as race, sex, age, and lifestyle. Thus, more rigorous human studies are required to understand the neuroprotective effect of tea. A number of laboratory experiments demonstrated the benefits of green tea and green tea catechins (GTCs), such as epigallocatechin gallate (EGCG), and proposed action mechanisms. The targets of GTCs include the abnormal accumulation of fibrous proteins, such as Aβ and α-synuclein, inflammation, elevated expression of pro-apoptotic proteins, and oxidative stress, which are associated with neuronal cell dysfunction and death in the cerebral cortex. Computational molecular docking analysis revealed how EGCG can prevent the accumulation of fibrous proteins. These findings suggest that GTCs have the potential to be used in the prevention and treatment of neurodegenerative diseases and could be useful for the development of new drugs.

Matrix metalloproteinases (MMPs) are zinc-dependent metalloproteinases that play important roles in a variety of diseases, including cancer, cardiovascular disease, diabetes, obesity, and brain diseases. Dietary polyphenols are thought to have a variety of beneficial effects on these diseases characterized by inflammation. Clinical studies have demonstrated that MMPs are in most cases upregulated in various inflammatory diseases, including osteoarthritis, rheumatoid arthritis, inflammatory bowel disease, and Alzheimer’s disease. Studies using patient-derived human samples, animal studies, and cellular experiments have suggested that polyphenols may be beneficial against inflammatory diseases by suppressing MMP gene expression and enzyme activity. One important mechanism by which polyphenols exert their activity is the downregulation of reactive oxygen species that promote MMP expression. Another important mechanism is the direct binding of polyphenols to MMPs and their inhibition of enzyme activity. Molecular docking analyses have provided a structural basis for the interaction between polyphenols and MMPs and will help to explore new polyphenol-based drugs with anti-inflammatory properties.

Most cell-based and animal experiments have shown that green tea catechins (GTC) exhibit various health benefits. In human experimental and epidemiological studies, there are conflicting results, and more precise investigations are required. One of the most effective ways to prove beneficial health effects in humans might be clinical intervention studies. Polyphenon^®E was developed as a standardized GTC preparation, which was approved by Food and Drug Administration of US in 2006 as a medication to treat genital warts (Veregen^® or sinecatechins). Positive efficacy of Polyphenon^®E/sinecatechins/Veregen^® (PSV) on anogenital warts has been demonstrated in several epidemiological studies and there have been several case reports to show the clinical effectiveness of PSV. In addition, several studies have provided evidence to suggest that PSV is effective in other human papillomavirus (HPV)-related diseases, although some studies failed to show such effects. Since (−)-epigallocatechin gallate (EGCG) is the major component of PSV, the mechanism of the action of PSV might be deduced from that of EGCG. The microarray analysis of the biopsy samples from the patients suggested that apoptosis induction and the downregulation of inflammation are involved in the mechanism of the action of PSV in the clearance of anogenital warts. Cell-based and animal experiments using PSV also demonstrated effects similar to those elicited by EGCG, explaining how PSV works to induce apoptosis and exert anti-inflammatory actions in HPV-related diseases. Future studies would clarify what kinds of diseases respond effectively to PSV, showing health benefits of GTC and EGCG in humans.

Epidemiological studies have shown that the consumption of green tea has beneficial effects against cancer. Basic studies have provided evidence that epigallocatechin gallate (EGCG) is a major contributor to these effects. Matrix metalloproteinases (MMPs) are zinc-dependent metalloproteinases with the ability to degrade the extracellular matrix proteins and are involved in various diseases including cancer in which MMPs have a critical role in invasion and metastasis. In this review, we discuss the effects of EGCG on several types of MMPs in the context of its anticancer activity. In the promoter region, MMPs have binding sites for at least one transcription factor of AP-1, Sp1, and NF-κB, and EGCG can downregulate these transcription factors through signaling pathways mediated by reactive oxygen species. EGCG can also decrease nuclear ERK, p38, heat shock protein-27 (Hsp27), and β-catenin levels, leading to suppression of MMPs’ expression. Other mechanisms by which EGCG inhibits MMPs include direct binding to MMPs to prevent their activation and downregulation of NF-κB to suppress the production of inflammatory cytokines such as TNFα and IL-1β. Findings from studies on EGCG presented here may be useful in the development of more effective anti-MMP agents, which would give beneficial effects on cancer and other diseases.

The hydrogen molecule (H₂), which has low redox potential, is produced by colonic fermentation. We examined whether increased H₂ concentration in the portal vein in rats fed high amylose maize starch (HAS) helped alleviate oxidative stress, and whether the transplantation of rat colonic microbiota with high H₂ production can shift low H₂-generating rats (LG) to high H₂-generating rats (HG). Rats were fed a 20% HAS diet for 10 days and 13 days in experiments 1 and 2, respectively. After 10 days (experiment 1), rats underwent a hepatic ischemia–reperfusion (IR) operation. Rats were then categorized into quintiles of portal H₂ concentration. Plasma alanine aminotransferase activity and hepatic oxidized glutathione concentration were significantly lower as portal H₂ concentration increased. In experiment 2, microbiota derived from HG (the transplantation group) or saline (the control group) were orally inoculated into LG on days 3 and 4. On day 13, portal H₂ concentration in the transplantation group was significantly higher compared with the control group, and positively correlated with genera Bifidobacterium, Allobaculum, and Parabacteroides, and negatively correlated with genera Bacteroides, Ruminococcus, and Escherichia. In conclusion, the transplantation of microbiota derived from HG leads to stable, high H₂ production in LG, with the resultant high production of H₂ contributing to the alleviation of oxidative stress.