Protective effects of green tea polyphenols against subacute hepatotoxicity induced by microcystin-LR in mice
Introduction
The monocyclic heptapeptides microcystins (MCs) are the most common and abundant cyanotoxins and pose a world health threat to humans and animals (de Figuereido et al., 2004). MCs are characterized as monocyclic heptapeptide with over 80 different MCs isoforms identified (WHO, 2004), but the most frequent and commonly studied isoform is microcystin-LR (MC-LR), followed by MC-RR, MC-YR and MC-LA.
MCs can strongly and irreversibly inhibit the activities of several serine/threonine protein phosphatases in hepatocytes, including PP1 and PP2A. This subsequently induces various responses leading to the disruption of normal cell metabolism and functions, such as protein hyper-phosphorylation, morphological changes of hepatocytes and tumour promotion (Nishiwaki-Matsushima et al., 1992). It is well known that MCs are responsible for illness and death not only in domestic and wildlife animals but also in humans (Guzman and Solter, 1999, Jochimsen et al., 1998). At present, the exact mechanisms by which MCs induce hepatotoxicity and tumour promotion have not been fully elucidated. Several pieces of evidences strongly suggest that oxidative stress may play a significant role in the pathogenesis of MCs-induced toxicity in mammals (Ding and Ong, 2003). Previous studies have shown that intraperitoneal injection of MC-LR resulted in a decrease in the antioxidant enzymes and induced elevated lipid peroxidation (LPO) in serum and relative organs in vivo (Moreno et al., 2005, Guzman and Solter, 1999), which may be related to changes in the antioxidant defense system.
Because of the rapid, irreversible and severe damage to the liver caused by MCs, therapy is likely to have little or no value, and effective prophylaxis is critical. In spite of the potential human hazards associated with MCs, very little work has been done on development of effective chemoprotectants or antidotes against these toxins. GTP, the main components of green tea used worldwide as a popular beverage that exert a wide range of biochemical and pharmacological effects, were numerously reported in the past decades that have significant anti-oxidative, anti-mutagenic and anti-carcinogenic activities. These results suggest that GTP might be a useful cancer chemopreventive agent in the human population (Lung et al., 2002, Yang et al., 1999). The main components of GTP are catechins, which have a polyphenol structure, including [(−)-epigallocatechin-3-gallate (EGCG)], [(−)-epigallocatechin (EGC)], [(−)-epicatechin-3-gallate (ECG)] and [(−)-epicatechin (EC)], all these catechins have strong antioxidant activity (Rice-Evans et al., 1996, Dufresne and Farnworth, 2001, Higdon and Frei, 2003) and are considered as potent scavengers of ROS, such as superoxide, hydrogen peroxide, hydroxyl radicals and nitric oxide produced by various chemicals (Guo et al., 1996, Jovanovic et al., 1995, Khan et al., 1992, Schroeder et al., 2003). The antioxidant activity increased in the following order: EC < ECG < EGC < EGCG. It has been demonstrated that the chemically induced lipid peroxidation in animal's liver and kidney could be inhibited by the intake of tea catechins (Sano et al., 1995).
Considering the diverse functional properties of GTP and the lack of studies on chemoprotectants against MCs-induced toxicity, we therefore undertook the present study to demonstrate whether subacute exposure to MC-LR in mice could modify the antioxidant defense system and induce oxidant damages. In addition, we evaluated the chemopreventive potential benefits of GTP pre-treatment prior to MC-LR and investigated the mechanisms of GTP protection effects.
Section snippets
Chemicals
The cyanobacterial toxin MC-LR (95%) was obtained from Alexis Biochemicals (Switzerland). GTP chemical was obtained from Tianbao biochemicals Co. Ltd., China (Lot# 20041030), among which the main components were 55% of EGCG, 17% of EGC, 14% of ECG, 10% of EC and 0.5% of caffeine, the other ingredients included amino acids (1%), sugars (0.5%), ash (0.5%), water (0.5%), minor elements, etc. GSH, SOD and MDA detection kits, in situ cell apoptosis detection kit (TUNEL) and Bcl-2 SABC detection kit
General observations
No mortality associated with MC-LR and GTP administration was observed throughout the experimental period and the behavior of the MC-LR-treated mice could not be distinguished from that of the controls and GTP pre-treated mice in any of the groups. However, The setting dose of MC-LR exposure has caused a subtle decrease in food consumption, subsequently sufficient to cause detectable weight loss.
Data for initial and final body weights, absolute and relative liver weights are summarized in Table
Discussion
Increasing evidences support the hypothesis that MCs-induced tissue damage may be a consequence of oxidative stress (Ding and Ong, 2003). In the present study, hepatotoxicity was observed either histologically or by clinical chemistry after MC-LR intoxication and the antioxidant defense system is modified by MC-LR treatment, suggesting the role of ROS in the pathogenesis of MC-LR-induced toxicity. We demonstrated that GTP remarkably protected mice from MC-LR-induced oxidative stress and liver
Acknowledgements
The authors would like to thank Dr. Jun-Yan Hong (School of Public Health/Environmental and Occupational Health Sciences Institute University of Medicine and Dentistry of New Jersey, USA) for critically reviewing this manuscript and kindly giving some precious advice. This work was supported by grants from National Natural Science Foundation of China (Grant No. 30630056), Chinese National Program for Key S&T Project (Grant No. 2003BA869C) and Program of S&T (Chongqing) Project (Grant No.
References (40)
- et al.
Studies on oxidative damage induced by cyanobacteria extract in primary cultured rat hepatocytes
Environ. Res.
(1998) - et al.
Calpain activation after mitochondrial permeability transition in microcystin-induced cell death in rat hepatocytes
Biochem. Biophys. Res. Commun.
(2002) - et al.
Role of oxidative stress and mitochondrial changes in cyanobacteria-induced apoptosis and hepatotoxicity
FEMS Microbiol. Lett.
(2003) - et al.
A review of latest research findings on the health promotion properties of tea
J. Nutr. Biochem.
(2001) - et al.
Determination of aldehydic lipid peroxidation products: malonaldehyde and 4-hydroxnonenal
Methods Enzymol.
(1990) - et al.
Studies on protective mechanisms of four components of green tea polyphenols against lipid peroxidation in synaptosomes
Biochim. Biophys. Acta
(1996) - et al.
Comparative toxicity evaluation of cyanobacterial cyclic peptide toxin microcystin variants (LR, RR, YR) in mice
Toxicology
(2003) - et al.
Hepatic and renal pathology of intraperitoneally administred microcystin-LR in rainbow trout (Oncorhynchus mykiss)
Toxicon
(1996) - et al.
Anti-proliferative and differentiation-inducing activities of the green tea catechin epigallocatechin-3-gallate (EGCG) on the human eosinophilic leukemia EoL-1 cell line
Life Sci.
(2002) - et al.
Superoxide dismutase. An enzymic function for erythrocuprein (hemocuprein)
J. Biol. Chem.
(1969)