Epigallocatechin Gallate for Management of Heavy Metal-Induced Oxidative Stress: Mechanisms of Action, Efficacy, and Concerns

In this review, we highlight the effects of epigallocatechin gallate (EGCG) against toxicities induced by heavy metals (HMs). This most active green tea polyphenol was demonstrated to reduce HM toxicity in such cells and tissues as testis, liver, kidney, and neural cells. Several protective mechanisms that seem to play a pivotal role in EGCG-induced effects, including reactive oxygen species scavenging, HM chelation, activation of nuclear factor erythroid 2-related factor 2 (Nrf2), anti-inflammatory effects, and protection of mitochondria, are described. However, some studies, especially in vitro experiments, reported potentiation of harmful HM actions in the presence of EGCG. The adverse impact of EGCG on HM toxicity may be explained by such events as autooxidation of EGCG, EGCG-mediated iron (Fe3+) reduction, depletion of intracellular glutathione (GSH) levels, and disruption of mitochondrial functions. Furthermore, challenges hampering the potential EGCG application related to its low bioavailability and proper dosing are also discussed. Overall, in this review, we point out insights into mechanisms that might account for both the beneficial and adverse effects of EGCG in HM poisoning, which may have a bearing on the design of new therapeutics for HM intoxication therapy.

Function of Green Tea Catechins in the Brain: Epigallocatechin Gallate and its Metabolites

Over the last three decades, green tea has been studied for its beneficial effects, including anti-cancer, anti-obesity, anti-diabetes, anti-inflammatory, and neuroprotective effects. At present, a number of studies that have employed animal, human and cell cultures support the potential neuroprotective effects of green tea catechins against neurological disorders. However, the concentration of (−)-epigallocatechin gallate (EGCG) in systemic circulation is very low and EGCG disappears within several hours. EGCG undergoes microbial degradation in the small intestine and later in the large intestine, resulting in the formation of various microbial ring-fission metabolites which are detectable in the plasma and urine as free and conjugated forms. Recently, in vitro experiments suggested that EGCG and its metabolites could reach the brain parenchyma through the blood–brain barrier and induce neuritogenesis. These results suggest that metabolites of EGCG may play an important role, alongside the beneficial activities of EGCG, in reducing neurodegenerative diseases. In this review, we discuss the function of EGCG and its microbial ring-fission metabolites in the brain in suppressing brain dysfunction. Other possible actions of EGCG metabolites will also be discussed.

Beneficial effects of (-)-epigallocatechin gallate on ischemia-reperfusion testicular injury in rats

BACKGROUND/PURPOSE

The purpose of the study was to determine the effect of (-)-epigallocatechin gallate (EGCG) on testicular ischemia-reperfusion injury in rats.

METHODS

Forty male Wistar rats were assigned to 5 groups. A sham operation was performed on the animals in group 1. In group 2, after 4 hours of unilateral testicular ischemia, 4 hours of testicular reperfusion was performed with EGCG administered 1 hour before reperfusion. In group 3, the same surgical procedure as in group 2 was performed, but without EGCG. Serum superoxide dismutase activity, creatine kinase, and lactate dehydrogenase were then measured in blood samples from groups 1 to 3. In group 4, after 4 hours of unilateral testicular ischemia, testicular reperfusion was performed. In group 5, the same procedure as in group 4 was performed, but with EGCG administered 1 hour before reperfusion. For groups 4 and 5, bilateral orchiectomy was performed for histologic examination 4 weeks after reperfusion was started.

RESULTS

Serum superoxide dismutase activity was significantly higher in group 2 than in group 3. The ratios of bilateral testicular weight, mean seminiferous tubule diameter, and germinal epithelial cell thickness were significantly higher in group 5 than in group 4.

CONCLUSIONS

Therapy with EGCG before reperfusion might exert protective effects via antioxidant activities in a rat experimental model of testicular ischemia-reperfusion injury.

Chitosan nanoparticles enhance the plasma exposure of (-)-epigallocatechin gallate in mice through an enhancement in intestinal stability

The green tea catechin (-)-epigallocatechin gallate (EGCG) has attracted significant research interest due to its beneficial therapeutic effects, which include anti-oxidant, neuro-protective and anti-cancer effects. However, the therapeutic potential of EGCG following oral consumption is limited by its poor absorption. To address this issue, EGCG has been encapsulated in chitosan-tripolyphosphate nanoparticles (CS NPs) and the oral absorption of EGCG evaluated in Swiss Outbred mice. Administration of the CS NPs enhanced the plasma exposure of total EGCG by a factor of 1.5 relative to an EGCG solution, with plasma AUC((0-5 h)) values of 116.4±4.1 and 179.3±10.8 nM.h (mean±s.d., n=3-5) for the EGCG solution and CS NPs, respectively. Associated with the increased plasma exposure of EGCG was an enhancement in concentrations of EGCG in the stomach and jejunum of mice following CS NP administration. A 2.3-fold increase in the apparent exposure of EGCG to the jejunum (AUC(j)) was observed following CS NP encapsulation, with AUC(j(0-5 h)) values of 5.3±1.1 and 12.3±1.5 μM.h (mean±s.d., n=3-5) for the EGCG solution and CS NPs, respectively. The enhanced exposure of EGCG to the jejunum was likely responsible for the increased plasma concentrations of EGCG. The findings from this study suggest that CS NPs may be a useful approach for enhancing oral delivery, and therapeutic application, of EGCG in a number of disease conditions.

Assessment of plasma concentrations of (-)-epigallocatechin gallate in mice following administration of a dose reflecting consumption of a standard green tea beverage

The plasma exposure of (-)-epigallocatechin gallate (EGCg) is typically assessed following administration of EGCg at doses equivalent to the consumption of at least 10 cups of green tea in one sitting. This study determines the plasma concentrations of EGCg in mice following administration of a dose reflecting typical consumption of one standard green tea beverage. Swiss Outbred mice were orally administered 0.76mg/kg EGCg, and using a validated HPLC method, the Cmax of un-conjugated and total EGCg was determined to be 31.5±3.3 and 34.3±2.0nM, respectively (mean±s.d., n=3-5). The area under the plasma concentration versus time curve for un-conjugated and total EGCg was 114.3±4.1 and 116.4±4.1nM·h, respectively (mean±s.d., n=3-5). To minimise potential ex vivo plasma degradation, a novel stabilizing solution of 20mM ascorbic acid (AA) and 13mM tris[2-carboxyethyl]phosphine hydrochloride (TCEP) was employed. Comparative evaluation of the efficacy of the 20mM AA and 13mM TCEP stabilizing solution to the commonly used stabilizing solution of 114mM AA and 0.13mM Na2EDTA, indicated that the AA/TCEP solution provided significantly greater (p<0.05) protection to EGCg than the AA/Na2EDTA stabilizing solution. Overall, this study demonstrates that plasma concentrations of EGCg are in the low nM range following oral administration to mice at a dose reflecting the consumption of a standard green tea beverage. In addition, a novel stabilizing solution has been identified which may be useful in stabilizing plasma samples obtained from pharmacokinetic studies.

Effect of dietary Polyphenon E and EGCG on lung tumorigenesis in A/J Mice

PURPOSE

To compare the chemopreventive efficacy of Polyphenon E (Poly E), (-)-epigallocatechin-3-gallate (EGCG) and Polyphenon E without EGCG (Poly E-EGCG) on the development of benzo(a)pyrene (B(a)P)-induced lung tumors in A/J mice.

METHODS

Female A/J mice were given a single intraperitoneal injection of B(a)P (100 mg/kg body weight). One week after B(a)P injection, animals received AIN-76A purified powder diet containing 0.975% (wt/wt) EGCG, 0.525% (wt/wt) Poly E-EGCG or 1.5% (wt/wt) Poly E for 24 weeks or control diet with no additives.

RESULTS

Poly E treatment significantly decreased tumor multiplicity by 52% and tumor load by 64%, while EGCG and Poly E-EGCG did not significantly inhibit lung tumor multiplicity. EGCG was more stable in a complex mixture (Poly E) than as a pure compound.

CONCLUSION

EGCG was ineffective when administered by diet likely due to its instability. Thus, EGCG's efficacy on mice lung tumorigenesis requires the presence of other tea catechins.