Catechin

General information

Cianidanol is an antioxidant flavonoid that occurs especially in woody plants. It is one constituent of green tea. It has immunomodulatory properties, including effects on T lymphocytes and killer cells [1].

Of 40 patients with chronic active hepatitis, 22 took cianidanol 3 g/day and 18 took placebo [2]. Adverse reactions to cianidanol were fever (n = 4), hemolysis (n = 1), and urticaria (n = 1).

Serious adverse reactions were not observed when cianidanol was used to treat HBe-antigen-positive chronic hepatitis in 338 patients [3]. The only adverse reaction of note that appeared to be drug-related was transient pyrexia in 13, necessitating withdrawal of therapy in eight. Four patients also had a skin eruption.

Sources of Catechins

As green tea is the most common aromatic beverage around the globe, many epidemiological studies correlated the tea consumption and decreased risk of various metabolic disorders as well as mortality in a dose-dependent fashion. Eighty percent of all polyphenols present in green tea are contributed by catechins. Mainly, EGCG, EGC, ECG, and EC are found in green tea. EGCG is the most abundant and the most pharmacologically potent catechin found in green tea [12].

Catechin is present in many dietary products, plants, fruits (such as apples, blueberries, gooseberries, grape seeds, kiwi, strawberries), green tea, red wine, beer, cacao liquor, chocolate, cocoa, etc. Tea and red wine are some of the most popular beverages in the world. The antioxidant action of catechin is well-established by various in vitro, in vivo and physical methods. Catechin affects the molecular mechanisms involved in angiogenesis, extracellular matrix degradation, the regulation of cell death, and multidrug resistance in cancers and related disorders. A positive correlation between green tea consumption and cardiovascular health due to several actions such as antioxidative, antihypertensive, anti-inflammatory, antiproliferative, antithrombogenic, and anti-hyperlipidemic etc., is well established based upon epidemiological and experimental studies. Clinical studies have shown the beneficial effects of catechin due its antioxidant action.

Summary Points

20.5.1 Catechin

Catechin (18) is a naturally occurring flavonol and a well-demonstrated antioxidant phytochemical originally derived from catechu, which is the tannic juice or boiled extract of Acacia catechu L.f (Fabaceae) [78]. Catechin is a constituent of several medicinal plants found throughout Africa such as Ficus mucuso [79], Ficus gnaphalocarpa [80], Ficus cordata (Moraceae) [81] and Khaya grandifoliola C.D.C. (Meliaceae) [82] harvested in Cameroon, Holothuria atra (Holothuriidae) [83], Thalassodendron ciliatum (Cymodoceaceae) [84] from Egypt, Psidium guajava (Myrtaceae) [85], Acalypha wilkesiana “Godseffiana” Muell Arg (Euphorbiaceae) [86] from Nigeria, Terminalia sericea (Combretaceae) [87], Peltophorum africanum (Fabaceae) [88] and Guibourtia coleosperma (Fabaceae) [89] from South Africa. Compound 18 is a pharmacologically active compound found to induce longevity in the nematode worm Caenorhabditis elegans [90], to inhibit intestinal tumor formation in mice [91], to inhibit the oxidation of low-density lipoprotein [92], to suppress expression of Kruppel-like factor 7 [93], to show an enhancement of the antifungal effect of amphotericin B against Candida albicans [94] and to prevent human plasma oxidation [95]. Transcriptomic studies also indicated that 18 could reduce atherosclerotic lesion development in apo E-deficient mice [96]. It was demonstrated that catechin-like compounds were strong therapeutic candidates for protection against the cognitive decline caused by the HIV. In fact, epicatechin, EGCG (19) and other catechin flavonoids may protect against neurotoxic oxidative stress caused by the HIV-Tat protein [97]. Epicatechin was found able to cross the blood–brain barrier and activate brain-derived neurotrophic factor (BDNF) pathways [97], suggesting its neuroprotective effects. Compound 18 was reported as histidine decarboxylase inhibitor, inhibiting the conversion of histidine to histamine suggesting to have beneficial effects through reduction of potentially damaging, histamine-related local immune response [98]. This phytochemical is also a selective monoamine oxidase inhibitor (MAOI) of type MAO-B, showing its ability to reduce the symptoms of Parkinson and Alzheimer diseases [99].

5.2 Catechins

Tea, produced from the leaves of Camellia sinensis, is one of the most widely consumed beverages in the world [104]. It has been postulated that green tea polyphenols, in particular EGCG, modulate cellular signal transduction pathways associated with the prevention of several diseases, including cancer, diabetes mellitus, and cardiovascular diseases (reviewed in Refs. [105,106]). Green tea polyphenols, especially EGCG, received much attention over the past few years as a potential cancer chemopreventive. Cytotoxic and genotoxic effects of these compounds were also reported [107–111]. Interactions of catechins with topoisomerase isoenzymes seem to play a crucial role for the observed impact on DNA integrity.

Several studies showed that the green tea catechins ECG and EGCG do not affect topoisomerase activity as analyzed by the DNA relaxation assay using pBR322 plasmid DNA and calf thymus topo1 as assay partners [112], or only at higher concentrations (≤ 500 μM) using purified topo1 from the human colon carcinoma cell lines HCT 116, VACO 241, or SW 480 [113]. In a further investigation, EGCG, ECG, and EGC exhibit inhibitory properties on calf thymus topo1 with IC₅₀ values of 5, 136, and 475 μM, respectively, whereas EC does not affect topo1 activity at concentrations up to 1 mM [114]. Overall, these results implicate that green tea catechins are only weak inhibitors of topo1. However, the substitution of a galloyl group at the 3 position and/or the hydroxyl group at the 3′ position indicates crucial structural characteristics for the observed topo1 inhibitory properties [113,114]. Using an immunofluorescence technique that employs specific antibodies to detect topo1–DNA intermediates in cells, the so-called TARDIS assay, Lopez-Lazaro et al. [115] show that EGCG (100 μM) potently increase the amount of topo1/DNA intermediates in K562 leukemia cells after 24 h of incubation.

Additionally, green tea catechins potently inhibit topo2 activity [80,114,116–118]. Further studies elucidated the underlying mechanism of action and characterized EGCG as a topo2 poison, without differentiating between 2α and 2β isoforms [112]. Recent studies suggested that the clinical effects of topo2 poisons may be related to individual enzyme isoforms, with the chemotherapeutic properties being attributed primarily to topo2α and the leukemogenic properties and off-target toxicity primarily mediated by topo2β [97,119]. Bandele and Osheroff [120] show that EGCG potently poisons both topo2α and topo2β. Furthermore, they characterize EGCG as topo2 poison with a redox-dependent mechanism of action that appears to act by covalently adducting the enzyme. These results are in line with the reported stabilization of topo2α and topo2β intermediates in K562 cells incubated with 100 μM EGCG [115]. EGCG was the first polyphenol which was reported to act as a covalent topoisomerase poison. Within the class of green tea polyphenols, EGC has also been characterized as a covalent topo2 poison in contrast to ECG and EC which do not affect topo2-induced DNA cleavage [83,120]. In summary, the activity of green tea catechins by targeting human type II topoisomerases is dependent on the presence of the three hydroxyl groups on the B-ring. The authors suggest that in a redox mechanism, the phenolic B-ring is converted to a reactive quinone. The galloyl group is reported as a second prominent structural characteristic that contributes to topo2 inhibitory effects because EGC is found to be less potent compared to EGCG. EGCG is an ester of EGC and gallic acid (GA). Lopez-Lazaro et al. [115] show that GA and pyrogallol (PG), the latter being the only shared structural feature between EGCG and GA, also potently increase topo2–DNA complexes. This indicates that the PG moieties of EGCG and GA contribute to the observed EGCG and GA-mediated cleavable complex formation [115].

In literature, it was extensively discussed that EGCG, EGC, and GA potently generate hydrogen peroxide (H₂O₂) in cell culture medium [109,121–123]. Therefore, a potential contribution of H₂O₂ to the observed topoisomerase inhibitory properties of these compounds has to be taken into account. In the presence of catalase, neither EGCG, EGC, GA, nor PG induces topoisomerase II/DNA cleavable complex formation, suggesting that H₂O₂ mediates these cellular activities [115].

It has to be mentioned that the natural source of catechins is green tea and that humans do not usually consume a single compound but rather a mixture of catechins within their diet. A consumer-relevant complex green tea extract, rich in catechins, also exhibits high potency to stabilize either the topo2α– or the topo2β–DNA intermediates. The authors demonstrate that EGCG contributes to the observed topoisomerase poisoning effects of the green tea extract. However, comparing the cleavage titrations of green tea extract and EGCG, it can be speculated that additional topo2-active compounds such as several bioactive flavonoids have to be present in the extract [120,126].

4.1 Antioxidant and Pro-oxidative Activities In Vitro and In Vivo

Catechins are well recognized as antioxidants, but they can also be pro-oxidants and generate ROS. ROS can alter the functions of cellular proteins, lipid, and nucleic acids and lead to different diseases [21]. Oxidative damages to DNA cause mutation and genomic instability, which are major contributing factors in the initiation, promotion, and progression of carcinogenesis [22]. However, there are also suggestions that decreasing the amounts of ROS promotes tumor progression [23,24]. Therefore, the antioxidant and pro-oxidative activities of catechins have the potential to affect cancer signaling, depending on the bioavailability of catechins and context of the cellular environment. Although the antioxidant activity of tea catechins is well established in vitro [5], such activity in vivo is only observed under circumstances when the animals are under oxidative stress. For example, EGCG administration has been found to decrease the levels of lipid peroxidation and protein carbonylation in old rats, but not in young rats [25]. In animal models for carcinogenesis, ROS are induced by the treatment with carcinogens, and EGCG has been demonstrated to reduce the formation of 8-hydroxydeoxyguanosine (8-oxo-dG), a well-established marker for oxidative DNA damage that can mispair to induce mutations [26]. As endogenously formed ROS are important in promoting carcinogenesis, tea polyphenols may have important roles in quenching these species at different stages of carcinogenesis. In human studies, administration of green tea to smokers for 4 weeks has been shown to significantly reduce the number of 8-oxo-dG-positive cells [27]. Such antioxidant actions of tea catechins may be important in the prevention of carcinogenesis.

Tea catechins can be auto-oxidized to generate ROS in cell culture medium and cause cell death [7,28]. After entering the cells, EGCG may also induce the production of ROS in the mitochondria. In our studies, oral administration of EGCG to mice bearing human lung cancer H1299 cell xenograft tumors inhibited tumor growth, enhanced tumor cell apoptosis, and produced ROS in the tumor cells [29]. The observed ROS production in tumor cells is probably due to the lack of sufficient antioxidant enzymes in H1299 cells. It remains to be demonstrated whether the production of ROS is responsible for the induction of apoptosis in vivo. At modest doses (e.g., 0.5% EGCG in the diet), although increased levels of 8-oxo-dG and phosphorylated histone 2A variant X were seen in xenograft tumors, 8-oxo-dG production and toxicity were not observed in the liver, kidney, and other organs of the host mice [29]. At high doses (e.g., 750 mg/kg, i.g.), however, hepatotoxicity and ROS were observed [30]. These toxic responses are probably similar to the reported liver toxicity in individuals who took excessive amounts of tea extracts in dietary supplements used for the purpose of weight reduction [31,32].

Cellular ROS may also activate the nuclear factor erythroid 2-related factor 2 (Nrf2)-mediated signaling pathways to induce cytoprotective enzymes [33]. For example, oral gavage of EGCG (200 mg per kg) to C57BL/6J mice upregulated gene expression of γ-glutamyltransferase, glutamate cysteine ligase, and hemoxygenase 1 in the liver and colon, which were most likely mediated by the activation of Nrf2 [34]. Similarly, human volunteers supplemented with 800 mg PPE per day for 4 weeks increased glutathione S-transferase P activity in lymphocytes [35]. In an intervention study in a high aflatoxin exposure area in China, supplementation with 500 or 1000 mg green tea polyphenols per day for 3 months increased the median urinary aflatoxin B1-mercapturic acid levels by more than 10-fold compared to baseline [36]. This result is likely due to the induction of glutathione S-transferase by EGCG. It appears that levels of ROS produced by moderate doses of tea polyphenols activate Nrf2 to reduce oxidative stress, but high doses of tea polyphenols can produce high levels of ROS, which induce toxicity [31]. Thus, the biological effect of EGCG depends on the dose used and the context of the biological systems.

Glossary

5.3 Antioxidant Property

The antioxidant properties of catechins are yet to be fully established. Cell culture studies have shown that catechins are effective free radical scavengers. Catechins may act as indirect antioxidants in concert with vitamins C and E and antioxidant enzymes, such as superoxide dismutase and catalase and add to the total antioxidant capacity of the plasma. Although catechins might be able to prevent the oxidation of vitamin E per se, however, ingestion of green tea catechins does not appear to modify the plasma status of vitamins E and C in vivo. Since low antioxidant activity of catechins has been attributed to their very low distribution and the form of metabolites in plasma as compared to other antioxidants, it has led to the hypothesis that catechins may have cell-signaling activity, for example, activation of Nrf2 and inhibition of NF-κB (Figure 33.2). Among the catechins, EGCG is the most-effective antioxidant, especially quenching ROS.