Effects of Longjing tea polyphenol extract on liver oxidative stress and metabolic enzymes in C57BL/6J mice
The liver is an important detoxification organ in the body, involved in regulating the metabolism of sugars, fats, and proteins, and is also the main site for the metabolism and transformation of exogenous substances. Therefore, liver function damage such as viral hepatitis, alcoholic fatty liver, liver fibrosis, cirrhosis, and liver cancer can have serious effects on the body. The liver contains a large number of phase I and phase II drug metabolizing enzymes, as well as various transport proteins. Phase I drug metabolizing enzymes are mainly CYP450 enzymes, responsible for oxidation, reduction, and hydrolysis processes, and can catalyze the metabolism of various exogenous substances such as drugs, toxins, and pre carcinogens. Phase II metabolic enzymes mainly include UDP glucuronosyltransferase (UGT) and sulfotransferase (SULT), which catalyze the covalent binding of phase I metabolic enzyme products with glucuronic acid, sulfuric acid, glutathione, etc., generating more water-soluble products, thereby facilitating the elimination of metabolic products. The ATP binding cassette transporter (ABC) mainly includes P-glycoprotein (P-gp) and multidrug resistance related proteins (MRPs), which belong to the same family and are widely present in various normal tissues and organs. They participate in the absorption, distribution, and excretion of drugs and internal/external substances, and have detoxification and defense protective effects.

The tea making process generally includes four steps: withering, fermentation, rolling, and drying. According to its processing method and fermentation degree, it is divided into white tea, yellow tea, green tea, black tea, black tea, and green tea. Tea has excellent antioxidant properties and has anti free radical activity both in vivo and in vitro. The physiological active substances in tea are mainly tea polyphenols, including catechins, anthocyanins, flavonoids, etc., which are also the main material basis for the antioxidant activity of tea. Green tea refers to unfermented tea leaves, and Longjing green tea is the first of the four famous teas in China, known as the “Queen of Green Tea”. The study of the physiological functions of active ingredients in green tea has become a research hotspot in recent years.

Our team’s previous research has shown that Longjing tea polyphenol extract has a regulatory effect on CYP450 in mouse liver, but its effects on phase II drug metabolizing enzymes and transporters are not yet clear. Therefore, based on previous research, this article explores the effects of Longjing tea polyphenol extract on the expression of phase II metabolic enzymes and transporters in the liver of C57BL/6J mice, as well as its resistance to oxidative stress, in order to provide experimental support for the deep development and utilization of Longjing tea polyphenol extract.

ALT and AST are two important indicators for measuring whether liver function is abnormal. Normally, ALT and AST are mainly present in the liver and have extremely low levels in the blood. But when liver function is abnormal, ALT and AST will be transferred from the liver to the blood, leading to an increase in ALT and AST levels and activity in the blood. The activity changes of ALT and AST in serum can indicate the health status of the liver. The results of this experiment showed that compared to the blank group, 75 mg/kg Longjing tea polyphenol extract could significantly reduce the level of ALT in mouse serum, 150 mg/kg Longjing tea polyphenol extract could significantly increase the level of AST in serum, and there was no significant difference in other dose groups. However, in clinical practice, a simple increase in AST does not necessarily indicate liver damage when ALT is normal, and pathological section analysis shows that Longjing tea polyphenols do not cause pathological damage to the liver of mice.

Oxidative stress on human cells and tissues is one of the causes of many diseases such as cancer and cardiovascular disease. To protect the body from oxidative damage, cells have a complete system of antioxidant enzyme defense enzymes. Mainly including SOD, CAT, GSH, GSH Px, etc. SOD converts superoxide anions into H2O2, while GSH Px and CAT catalyze H2O2 into H2O, thereby converting toxic superoxide anions and H2O2 into non-toxic H2O. Tea polyphenols are excellent natural antioxidants, rich in – OH groups, with good in vivo and in vitro antioxidant activity, and strong regulatory effects on oxidative stress. Longjing tea polyphenol extract had no significant effect on SOD activity in mouse liver, but could significantly increase GSH Px activity. 37.5mg/kg Longjing tea polyphenol extract could significantly increase CAT activity in liver. It can be seen that Longjing tea polyphenol extract can increase the ability of mouse liver to resist oxidative stress by regulating the content of CAT and GSH Px in mouse liver. For GSH, as the dosage of Longjing tea polyphenol extract increases, its content in the liver decreases, which may be due to the consumption of GSH by other components of Longjing tea polyphenols as an extract.

CYP450 is involved in the metabolism of endogenous/exogenous substances, including drugs, environmental compounds, etc., and has the characteristics of large individual differences and susceptibility to the influence of exogenous substances. 37.5mg/kg Longjing tea polyphenol extract can significantly increase the protein expression levels of CYP2E1/3A11 subtypes in mouse liver. Low dose Longjing tea polyphenol extract has a certain induction effect on CYP450. Phase II metabolic enzymes UGTs and SULTs are mainly distributed in the liver, metabolizing endogenous/exogenous substances into easily excreted water-soluble substances. 37.5mg/kg Longjing tea polyphenol extract significantly increased the protein expression levels of UGT1A6 and SULT1A1 in mouse liver, while 75 and 150 mg/kg Longjing tea polyphenol extract had no significant effect on the expression of these two enzymes. Indicating that low-dose Longjing tea polyphenol extract has a certain induction effect on phase II metabolic enzymes. MRP2 is mainly distributed on the basement membrane of human hepatocytes. It can regulate the damage of hepatocytes in obstructive jaundice, participate in the transport of various toxic compounds inside and outside cells, adjust the redistribution of substances inside cells, promote the secretion of bile, and increase the fat solubility of bile salts. 75 and 150 mg/kg Longjing tea polyphenol extracts can significantly increase the protein expression level of MRP2 in mouse liver, with potential protective effects on liver cells. P-gp is a common molecular pump that protects cells from the invasion of harmful foreign molecules. It is located on the cell membrane, can bind to drugs, and has ATPase activity. It can break down ATP to supply energy, pump drugs out of the cell, reduce drug concentration inside the cell, and make cells develop drug resistance. The 37.5 and 150 mg/kg Longjing tea polyphenol extracts showed a downregulation effect on the protein expression level of P-gp, while the 75 mg/kg Longjing tea polyphenol extract had no significant effect on the protein expression level of P-gp. This may be due to the metabolic products of Longjing tea polyphenols or their own excretion through P-gp. The review by Huang Xixi et al. confirmed that there is an interaction between EGCG in tea polyphenols and drug metabolizing enzymes and transporters, and its specific mechanism of action needs further confirmation.

Through this study, it can be seen that Longjing tea polyphenols can enhance the oxidative defense system function in the liver of mice, helping the body resist internal/external oxidative stress. Low dose Longjing tea polyphenols have a certain inducing effect on phase I metabolic enzymes, phase II metabolic enzymes, and transporters in the liver of mice. The regulation of antioxidant enzyme systems, phase II metabolic enzymes, and transporters involves the Nrf2 Keap1 pathway. The CYP450 enzyme system is regulated by the androgen receptor (CAR) and pregnane X receptor (PXR), and the effects of Longjing tea polyphenols on these potential pathways deserve further investigation.