This article was downloaded by: [University of Sydney] On: 02 October 2013, At: 10:54 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Critical Reviews in Food Science and Nutrition Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/bfsn20 Green Tea: Nature's Defense against Malignancies Masood Sadiq Butt a & Muhammad Tauseef Sultan a a National Institute of Food Science and Technology, University of Agriculture, Faisalabad Published online: 27 Apr 2009. To cite this article: Masood Sadiq Butt & Muhammad Tauseef Sultan (2009) Green Tea: Nature's Defense against Malignancies, Critical Reviews in Food Science and Nutrition, 49:5, 463-473, DOI: 10.1080/10408390802145310 To link to this article: http://dx.doi.org/10.1080/10408390802145310 PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions
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This article was downloaded by: [University of Sydney]On: 02 October 2013, At: 10:54Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House,37-41 Mortimer Street, London W1T 3JH, UK
Critical Reviews in Food Science and NutritionPublication details, including instructions for authors and subscription information:http://www.tandfonline.com/loi/bfsn20
Green Tea: Nature's Defense against MalignanciesMasood Sadiq Butt a & Muhammad Tauseef Sultan aa National Institute of Food Science and Technology, University of Agriculture, FaisalabadPublished online: 27 Apr 2009.
To cite this article: Masood Sadiq Butt & Muhammad Tauseef Sultan (2009) Green Tea: Nature's Defense against Malignancies,Critical Reviews in Food Science and Nutrition, 49:5, 463-473, DOI: 10.1080/10408390802145310
To link to this article: http://dx.doi.org/10.1080/10408390802145310
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) containedin the publications on our platform. However, Taylor & Francis, our agents, and our licensors make norepresentations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of theContent. Any opinions and views expressed in this publication are the opinions and views of the authors, andare not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon andshould be independently verified with primary sources of information. Taylor and Francis shall not be liable forany losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoeveror howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use ofthe Content.
This article may be used for research, teaching, and private study purposes. Any substantial or systematicreproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in anyform to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http://www.tandfonline.com/page/terms-and-conditions
MASOOD SADIQ BUTT and MUHAMMAD TAUSEEF SULTANNational Institute of Food Science and Technology, University of Agriculture, Faisalabad
The current practice of introducing phytochemicals to support the immune system or fight against diseases is based oncenturies old traditions. Nutritional support is a recent advancement in the domain of diet-based therapies; green tea andits constituents are one of the important components of these strategies to prevent and cure various malignancies. Theanti-carcinogenic and anti-mutagenic activities of green tea were highlighted some years ago suggesting that it could reducethe prevalence of cancer and even provide protection. The pharmacological actions of green tea are mainly attributed topolyphenols that includes epigallocatechin-3-gallate (EGCG), epicatechin, epicatechin-3-gallate, epigallocatechin. Greentea and its components effectively mitigate cellular damage arising due to oxidative stress. Green tea is supposed to enhancehumoral and cell-mediated immunity, decreasing the risk of certain cancers, and may have certain advantage in treatinginflammatory disorders. Much of the cancer chemopreventive properties of green tea are mediated by EGCG that inducesapoptosis and promotes cell growth arrest, by altering the expression of cell cycle regulatory proteins, activating killercaspases, and suppressing nuclear factor kappa-B activation. Besides, it regulates and promotes IL-23 dependent DNArepair and stimulates cytotoxic T cells activities in a tumor microenvironment. It also blocks carcinogenesis by modulatingthe signal transduction pathways involved in cell proliferation, transformation, inflammation and metastasis. The reviewis intended to highlight the chemistry of green tea, its antioxidant potential, its immunopotentiating properties and modeof action against various cancer cell lines that showed its potential as a chemopreventive agent against colon, skin, lung,prostate, and breast cancer.
Keywords green tea, nutritional support, oxidative stress, immunity, epigallocatechin-3-gallate, malignancies
INTRODUCTION
Nutritional support is a recent development, prior to whichthe value of food was recognized for its medicinal benefits asnutraceuticals. Nutraceuticals are widely accepted as an ad-junct to conventional therapies for enhancing general well-being. The value of such “alternative” therapy is now beingrediscovered by many researchers that support the use of tradi-tional remedies to cure maladies (Klein et al., 2000; Ramaaet al., 2006). Micronutrients like antioxidants, plant sterols,and flavonoids have shown several therapeutic potential againstvarious health risks (Messina and Messina, 2003; Thurnham,1999). Evidence has been provided that dietary phytochemicalsmay play important roles as chemopreventive or chemother-apeutic agents in the prevention of many diseases, possessesantimutagenic effects and indeed modulating and stimulating
Address correspondence to Masood Sadiq Butt. E-mail: [email protected]
the immune system (Raskin et al., 2002; Rates, 2001) that inturn results in normal functioning of the whole body. Variousplants and their constituents have shown beneficial therapeuticeffects, including anti-oxidant, anti-inflammatory, anti-cancer,and immunomodulatory effects and green tea is one of them(Miller et al., 2004; Huffman, 2003; Parab et al., 2003; Fong,2002).
The plant of tea (Camellia sinensis) has been grown inSoutheast Asia for thousands of years and now is cultivatedin more than 30 countries around the world. Its consumptionhas reached a point where it has become the second most com-monly consumed beverage worldwide. This popularity was dueto its characteristic aroma, flavor, and most influencing its healthbenefits (Ahmad et al., 1998; Harbowy and Balentine, 1997).The basic steps of manufacturing the various forms of teas(Black, Green, and Oolong) are similar, except in the devel-opment of their aroma and in the fermentation process, whichis dependent on the oxidation states of catechins present in tealeaves. It is estimated that about 2.5 million metric tons of teais manufactured annually (Hara, 2001; Katiyar and Mukhtar,1996).
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The term “green tea” refers to the product manufacturedfrom fresh tea leaves by steaming or drying at elevated tempera-tures with the precaution to avoid oxidation of the polyphenoliccomponents (Chow and Kramer, 1990). Polyphenol oxidase(PPO) is mainly responsible for catalyzed oxidation of tealeaf catechins (Wilson and Clifford, 1992). Oolong tea is semi-fermented while black tea is a nearly fully fermented productand their processing involves the oxidation reaction catalyzed byPPO (Obanda et al., 2001). These reactions favor condensationof catechins with orthoquinones to form theaflavins which sub-sequently reacts with gallic acid to form epitheaflavic acid andtogether all these products are termed as thearubigins (Karoriet al., 2007). Fermentation and enzymatic oxidations brings outthe difference in compositions of different types of tea.
Type of tea % share in consumption Countries
Black tea 78% Western countries and some Asian countriesGreen tea 20% Asian countries mainly Japan, China, Korea & IndiaOolong tea 2% Southeastern China
Like most herbs, the precise composition of green tea varieswith the geographic origin of the leaf, the time of harvest, andprocessing techniques. Dried tea leaves are mainly composedof polyphenols (10–25%), principally flavonols (including cat-echins), flavonoids, and flavondiols. The leaves also containplant alkaloids (about 4%) including caffeine, theobromine,and theophylline. Other constituents include proteins, carbo-hydrates, phenolic acids, minerals (including fluoride and alu-minum), and fiber (Yang and Landau, 2002; Dolby and Mitscher,1998; Kaegi, 1998; Mukhtar et al., 1994; Weisburger, 1992).
The health-promoting properties of the tea plant of-ten refers to active ingredients that includes polyphenols{epigallocatechin-3-gallate (EGCG), epicatechin, epicatechin-3-gallate, epigallocatechin}, purine alkaloids {caffeine (2.9–4.2%), theobromine (0.15–0.2%), theophylline (0.02–0.04%)},inorganic ions fluoride, potassium, aluminum, flavonoids (e.g.
Polyphenols account for the pungency and the unique fla-vor of green tea. Catechins and other polyphenols present ingreen tea are antioxidant and anti-inflammatory in nature andhave been shown to possess anticarcinogenic activity (Baligaand Katiyar, 2006). The biological activity of green tea is dueto different catechins and EGCG is identified as the principalantioxidant contributing approximately 30% of the total antiox-idant capacity of green tea and has been recognized as the majorand potentially effective chemopreventive agent present in greentea leaves (Stewart et al., 2005; Ahmad et al., 1998; Katiyar andMukhtar, 1996).
RADICAL SCAVENGING ACTIVITYIES
Reactive oxygen species can potentially lead to damage ofalmost all types of biological molecules including DNA, lipids,proteins, and carbohydrates (Valko et al., 2007). Oxidative stressresults after the excessive production of reactive oxygen speciesthat overrides the antioxidant capability of the target cells (Vinaet al., 2006; Opara, 2004). Outcomes are DNA damage, produc-tion of mutated tumor-suppressor genes, and inducing cell death(Farah, 2005; Kang et al., 1997). These pathological events areinvolved in cardiovascular, neurodegenerative, and carcinogenicprocesses (Mobley and Brueggemeier, 2004; Ferrari, 2004). Im-proved antioxidant defence results in reduced prevalence of dis-eases associated with oxidative stress (Blomhoff, 2004; Ji andPeterson, 2004).
The production of free radical depends upon various extrin-sic and intrinsic factors. Generally these free radicals have been
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Figure 1 Structures of polyphenols present in green tea.
implicated in playing a role in more than 100 diseases that alsoincludes cancer, atherosclerosis, and arthritis. Although humancells possess a large number of endogenous antioxidants butstill certain amounts of exogenous antioxidants are constantlyrequired to maintain an adequate level of antioxidants in orderto balance the reactive oxidative species in the human body.Antioxidants protect the body by neutralizing the free radicalsand donating one of their own electrons, thus ending the scav-enger reaction. They have been found to be quite successful inthe prevention of certain diseases for years especially cancer(Berger, 2005; Thomas, 1995; Frankel, 1984).
Isbrucker et al., (2006) examined the antioxidant activities ofwater extracts of the various teas including the reducing power,the 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavengingactivity, and the inhibition of hemolysis caused by 2,2′-azo-bis(2-amidinopropane) dihydrochloride (AAPH)-induced lipidoxidation in erythrocyte membranes and they supported the useof green tea as a potential antioxidant. The chemical, in vitroand biological assays, demonstrated green tea polyphenols asstrong antioxidants in their activity against iodophenol-derivedphenoxyl radicals, superoxide anion radicals, and lipid peroxi-dation in rat liver microsomes (Zhang and Shen, 1997; Katiyarand Mukhtar, 1996). Salah et al., (1995) reported that the teaextracts are powerful antioxidants, mainly owing to the pres-ence of (+)-catechin, (−)-epicatechin, (−)-epigallocatechin,(−)-epigallocatechin gallate (EGCG), and (−)-epicatechin gal-late.
EGCG is a valuable scavenger of reactive oxygen species andhas strong antioxidant activity (Norwood et al., 2006; Bagchi,
1999). It protects cellular damage by inhibiting DNA damageand oxidation of LDL and many putative health benefits of teaare presumed to be caused by its antioxidant effects. EGCG canreduce the inflammatory response associated with local tissueinjuries such as the hepatocellular necrosis in acute liver in-jury induced by carbon tetrachloride. The protective effects ofEGCG are due to its ability to decrease lipid peroxidation, ox-idative stress, and the production of nitric oxide (NO) radicalsby inhibiting the expression of iNOS. It also ameliorates theoverproduction of pro-inflammatory cytokines and mediators,reduces the activity of NF-kappa-B and AP-1, and the subse-quent formation of peroxynitrite with NO and reactive oxygenspecies (Tipoe et al., 2007).
Thus, green tea or EGCG effectively mitigates cellular dam-age by lowering the inflammatory reaction and reducing the lipidperoxidation and NO generated radicals leading to the oxidativestress. Green tea is proposed as a dietary supplement in the pre-vention of cardiovascular diseases and other diseases in whichoxidative stress and proinflammation are principal causes.
IMMUNOMODULATORY AND ANTI-INFLAMMATORYEFFECTS
Immunity is considered as the ability of an organism to fightagainst any abnormal function occurring within the body andthe immune system prevents infections and diseases by moder-ating malignant and foreign cells within the body (Schulenburget al., 2007). The human’s immune system is composed of
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organs like spleen and thymus while additionally lymph nodesand bone marrow also contribute to the proper immune function-ing by producing and storing specific immune cells (Schmid-Hempel, 2005). Green tea enhances humoral and cell-mediatedimmunity decreasing the risk of certain cancers and the riskof cardiovascular disease (Klein et al., 2000). Polyphenols arethe active ingredients in green tea that hold antioxidant, anti-inflammatory, and anti-carcinogenic properties and such healthbenefits of tea are mainly attributed to its polyphenols contents(Higdon and Frei, 2003; Katiyar et al., 2001).
Inflammation is an integral part of the immune system butsometimes becomes a causative agent in causing some dis-eases (Bystrianyk, 2005). Dona et al., (2003) provide molecularand cellular insights into the claimed beneficial properties ofgreen tea and indicated that epigallocatechin-3-gallate (EGCG)is potent anti-inflammatory compound with therapeutic poten-tial. Studies in animal models show that green tea polyphenolsdecrease inflammation. It has been reported that mice fed onthe extract of green tea polyphenols had decreased the tumornecrosis factor- (TNF) production in response to an injectionof lipopolysaccharide (LPS) and prevented death after admin-istration of a lethal dose of LPS (Yang et al., 1998). The in-gestion of a green tea polyphenol extract reduces joint diseasein mice with adjuvant-induced arthritis and their consumptionreduces disease activity in the autoimmune disease models likeinterleukin-2 deficient mice (Haqqi et al., 1999; Varilek et al.,1999). These studies suggest that green tea may have benefitsin treating inflammatory disorders.
Several chemicals are used to induce inflammatory responsesthat are further used to study anti-inflammatory properties ofvarious agents. Topical application of phorbol esters like TPAis one of them. These inflammatory responses include epider-mal hyperplasia, inflammation, increase in the number of darkbasal keratinocytes, and induction of epidermal ornithine de-carboxylase (ODC) activity followed by an increase in thelevels of polyamines. Further, ODC plays an essential rolein cell proliferation and differentiation. Induction mediatedby TPA is believed to be governed by cyclooxygenase andlipoxygenase catalyzed metabolites of arachidonic acid, specifi-cally prostaglandins and hydroxyeicosatetraenoic acids, respec-tively (Agarwal and Mukhtar, 1993; Agarwal and Mukhtar,1991). Several controlled studies involving TPA, topical ap-plication of green tea polyphenols (GTP) to mouse skin inhibitsTPA-mediated induction of epidermal ODC activity in a dose-dependent manner (Agarwal et al., 1993). GTP prevents TPA-induced oxygen radical-induced cytotoxicity, inhibits intercel-lular communication in normal human epidermal keratinocytes,and inhibits TPA-induced protein kinase-C activity (Ruch et al.,1989; Yoshizawa et al., 1987).
Topical application of GTP to mouse skin inhibits12-0-tetradecanoylphorbol-13-acetate and other skin tumor-promoter-caused induction of protein and mRNA expressionof the pro-inflammatory cytokines interleukin (1L)-1 α andTNF-α. Skin applications of green tea polyphenols inhibits UV-radiation-induced local and systemic suppression of contact
hypersensitivity and edema responses in C3H/HeN mice. Inmany in vitro studies, green tea polyphenols/crude extracts ofgreen tea have shown to have preventive effects on the systemconsidered essential in inflammatory processes (Ahmad et al.,1997; Katiyar and Mukhtar, 1996).
Skin application of GTP to SENCAR mice resulted in signif-icant protection against TPA-caused effects on cyclooxygenaseand lipoxygenase activities. Katiyar et al., (1992c) elaboratedthat prior application of GTP onto the dorsal skin of mice re-sults in significant inhibition of TPA-mediated epidermal edemaand hyperplasia. Pre-application of GTP can also protect signifi-cantly against TPA-induced hyperplasia in the ear skin and TPA-caused infiltration of polymorphonuclear leukocytes (Katiyaret al., 1993). Toll-like receptors (TLRs) play an important rolein recognition of microbial components and induction of innateimmunity. The microbial components trigger the activation oftwo downstream signaling pathways of TLRs; MyD88- and/orTRIF-dependent pathways leading to activation of NF-kappa-B. (-)-Epigallocatechin-3-gallate (EGCG), a flavonoid found ingreen tea, is known to inhibit NF-kappa-B activation induced bymany pro-inflammatory stimuli. Green tea flavonoids modulateMyD88- and TRIF-dependent signaling pathways of TLRs andsubsequent inflammatory target gene expression (Youn et al.,2006). Several studies have focused on potential mechanisms re-sponsible for the anti-inflammatory and anticancer effects. Onepotential mechanism of action is the inhibition of nuclear factor-κB activation. Nuclear factor-κB is an oxidative stress-sensitivetranscription factor that regulates the expression of a variety ofgenes important in cellular responses, including inflammation,innate immunity, and growth. EGCG decreased LPS-inducedTNFα production in the macrophage cell line RAW264.7 andperitoneal macrophages by blocking NF-κB activation (Yanget al., 1998). In a similar study, Lin and Lin (1997) suggestedthat EGCG inhibits LPS-induced inducible nitric-oxide synthasegene expression in mouse peritoneal macrophages by decreasingthe expression of the transcription factor, NF-κB.
MALIGNANCIES AND GREEN TEA
Cancer is generally considered as uncontrolled cell divisionthat results in the aggregation of cells to form tumors. There aremany factors which are involved in the pathogenesis of cancers,e.g. genetic mutations, smoking, heavy metal ingestion, andother pollution and indeed lack of proper diet. The immunesystem plays a crucial role and inflammation eventually causesaggregation of cells due to disturbances in signaling pathwaysthat are associated with the pathogenesis of cancers (Noonanet al., 2007).
The initial step in carcinogenesis involves the metabolic ac-tivation of chemical carcinogens by the P-450-dependent bio-transformation reaction (Mukhtar et al., 1991). Cytochrome P-450 is the major enzyme system responsible for the metabolismof procarcinogens to their DNA binding metabolites. This
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binding to DNA is considered essential for tumor initiation(Agarwal and Mukhtar, 1991). Inflammation is a key compo-nent in cancer insurgence that can promote tumor angiogenesiswhich is necessary for solid tumor growth. Controlling suchmechanisms of inflammation and controlled cell death are keytargets of most chemopreventive agents (Noonan et al., 2007).
Harmlessly disposing of cancer cells is one of the main con-siderations in chemotherapy (Chan et al., 1997). Cell death canoccur by either of two distinct mechanisms: necrosis (“acci-dental” cell death) and apoptosis (“programmed” cell death). Avariety of pharmacological and physiological agents can trig-ger the cascade of events that leads to apoptosis. The life spanof both normal and cancer cells within a living system is sig-nificantly affected by the rate of apoptosis. Chemopreventiveagents that could modulate apoptosis might affect the steady-state cell population (Fesus et al., 1995). Therefore, it can beassumed that the chemopreventive agents with proven effectsin animal tumor bioassay systems and human epidemiologyand the ability to induce apoptosis of cancer cells, may havewider implications for the management of cancer. Flavanols aregroups of chemicals that possess strong nucleophilic centers attwo positions. This property provides an opportunity for the fla-vanols to react with electrophilic carcinogenic species to formflavonol-carcinogen adducts that may result in the prevention oftumorigenesis.
Catechins and polyphenols from green tea show the anti-cancer activity. The anti-carcinogenic and anti-mutagenic ac-tivity of polyphenolic agents present in green tea were firstreported almost couple of decade ago (Colic and Pevelic, 2000;Khan et al., 1988; Wang et al., 1989a; b). Green tea has shown inexperimental animal studies, human cell line laboratory studies,and also epidemiologically in large human population studiesthat it could significantly reduce the risk, protect from or pre-vent several cancers, and improve treatment that includes thetreatment of the biliary tract, bladder, breast, and colon cancer(Takada et al., 2002; Kamat and Lamm, 2002; Berger et al.,2001; Sartippour et al., 2002; Hong et al., 2002). Green tea isalso effective in leukemia (Smith and Dou, 2001), liver, andlung cancer (Bertram and Bartsch, 2002; Fujimoto et al., 2002).Some other scientific studies reported its beneficial effects inesophageal, prostate, and skin cancer (Youn et al., 2006; Man-tena et al., 2005; Gupta et al., 2003; Proniuk et al., 2002; Liao,2001; Katiyar and Mukhtar, 1997). Much of the cancer chemo-preventive properties of green tea are mediated by EGCG andit has been assumed that it induce apoptosis and promote cellgrowth arrest by altering the expression of cell cycle regulatoryproteins, altering Bax/Bcl2 function, activating killer caspases,and suppressing nuclear factor kappa-B function. EGCG modu-lates the signal transduction pathways involved in cell prolifer-ation, transformation, inflammation, apoptosis, and metastasis(Khan et al., 2006; Na and Surh, 2006; Gupta et al., 2004).Although different hypotheses are presented for its chemopre-ventive mechanism but still needs the attention of researchers tocarry out joint research programs to capture its true mechanismof actions.
Colon Cancer
Several population-based studies have shown that green teahelps protect against colon cancer. For example, cancer ratestend to be low in countries such as Japan where people reg-ularly consume green tea. However, it is not possible to de-termine from these population-based studies whether green teaactually prevents cancer in people. Researchers also believe thatpolyphenols help kill cancerous cells and stop their progression.Emerging studies suggested that the polyphenols in green teamay play an important role in the prevention of colon cancer(Bushman, 1998).
There are several factors that are associated with the onsetand progression of colon cancer and several lines of mechanismhave been presented for the action of green tea or its activeingredients. EGCG treatment to colon cancer cells results instrong activation of AMPK, an inhibition of COX-2 expression.Increased expression of COX-2 appears to play an importantrole in the development of colorectal cancer (Shimizu et al.,2005). The activation of AMPK with the reduction of VEGF(vascular endothelial growth factor) and glucose transporter,Glut-1 in EGCG-treated cancer cells supports the regulatoryrole of AMPK in COX-2 expression in EGCG-treated cancercells (Hwang et al., 2007). EGCG inhibited COX-2 mRNA andprotein overexpression. The effect of EGCG on COX-2 expres-sion resulted in decreased COX-2 promoter activity via inhibi-tion of nuclear factor kappa-B activation (Lambert et al., 2006;Balavenkatraman et al., 2006). Peng et al., (2006) observed thatEGCG down-regulated the ERK1/2 and Akt pathways in coloncancer cells.
The transmembrane protein-tyrosine phosphatase DEP-1(density-enhanced phosphatase) is a potential candidate for tu-mor suppression in the colon epithelium. Upregulation of DEP-1expression, and in turn inhibition of cell growth and migra-tion may present another possible unrecognized mechanism ofchemoprevention and green tea polyphenols had the capacity toelevate transcription of endogenous DEP-1 mRNA and expres-sion of DEP-1 protein (Jeong et al., 2004).
EGCG reduces LPS-induced Ikappa-Balpha phosphoryla-tion, potently inhibited cell growth, and induces Caspase-3activity. These are important signaling pathways and EGCGimparts a potential biological function to cater to its chemo-preventive properties.
In another study, Chen et al., (2003) observed that EGCGinhibited HT-29 cell growth with an IC50 of approximately 100microM after 36 h treatment. Their observation also includesCaspase-3 and Caspase-9 activation, mitochondrial transmem-brane potential transition, and cytochrome c release and as anearly signaling event was activation of MAPKs. Inhibition ofc-Jun N-terminal kinase (JNK) pathway shows that the involve-ment of JNK in EGCG-induced cytochrome-c release and celldeath. Jia and Han, (2001) investigated the effect of green teaon 1,2-dimethylhydrazine (DMH)-induced aberrant crypt foci(ACF) formation in Wistar rats and concluded that green teadrinking inhibits ACF formation in rats, and such effects may
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relate to the suppression of cell proliferation in the intestinalcrypts.
Some studies on the effects of green tea on colon or rectalcancer produced conflicting results. EGCG when compared tothe chemotherapeutic drug of choice, 5-fluorouracil (5-FU) havesome comparable chemotherapeutic responses to the SW-626cell line (Zhang and Shen, 1997) which indicates that EGCGhas a similar response in decreasing cell number with 5-FU.Population-based studies showed decrease risk in those whodrink tea, while in some instances researchers showed increasedrisk. Further research is needed before researchers can recom-mend green tea for the prevention of colorectal cancer.
Skin Cancer
Skin cancer represents a major, and growing, public healthproblem. It has been estimated that more than one million newcases of skin cancers are diagnozed each year in the UnitedStates alone, which is equivalent to the incidence of malignan-cies in all other organs combined (Housman et al., 2003). Skin-tumor promotion is divided into stages known as stage I andstage II (Agarwal and Mukhtar, 1993). TPA is widely employedas tumor promoter in two-stage skin tumorigcnesis protocols.
Topical application or oral feeding of a polyphenolic fractionisolated from green tea, referred to as GTP, to SENCAR, CD-1and Balb/C mice results in significant protection against skin tu-morigenesis (Mukhtar et al., 1992, Katiyar et al., 1992b). Topicalapplication of GTP concurrently with each application of eitherTPA (anti-stage I) or mezerein (anti-stage II) for 7 d prior to thesingle application of 7,12-dimethylbenz(a)anthracene (DMBA)as the initiating agent followed by twice weekly applicationsof TPA results in significant protection against tumorigenesis(Wang et al., 1989a). Mukhtar et al., (1994) also reported thata topical application of GTP results in significant protectionagainst tumor formation in DMBA-initiated SENCAR mouse,in terms of both tumor multiplicity (42–50%) and tumor growth(43–54%) Experimental studies involving animals, GTP appli-cation results in lower tumor body burden such as decrease inthe total number of tumors per group, number of tumors per an-imal, tumor volume per mouse, and average tumor size. Topicalapplication of GTP to CD-1 mice inhibited TPA-induced tumorpromotion in DMBA-initiated skin and inhibits tumor promo-tion by TPA and other skin-tumor promoters such as teleocidinand okadaic acid (Katiyar et al., 1992c; Huang et al., 1992).
In another intervention it was demonstrated that GTP in-teract with BP-7,8-diol 9,10-epoxide-2 and their topical appli-cation prior to BP-7,8-diol 9,10-epoxide-2 treatment results ininhibition of skin tumor initiation (Khan et al., 1988). GTP orEGCG when given intraperitoneal inhibits tumor growth andcauses partial regression of established skin papillomas. Epi-dermal aryl hydrocarbon hydroxylase activity and epidermalenzyme-mediated binding of BP and DMBA to DNA was in-hibited by these polyphenols (Wang et al., 1989a; Wang et al.,1989b).
Ultraviolet B (UVB) radiation present in the solar spectrumis one of the major risk factors for skin cancer in humans (El-mets, 1991). Chronic oral feeding of 0.1% GTP in drinkingwater (w/v) to mice during the entire period of UVB exposurewas found to result in significantly lower tumor body burden ascompared to non -GTP-fed (Wang et al., 1991). In another study,it was shown that infusion of green tea extracts as a sole source ofdrinking water (125%, w/v) to mice afforded protection againstUVB radiation-induced intensity of red color and area of skinlesions, as well as UVB radiation-induced tumor initiation andtumor promotion (Wang et al., 1992a; b). Mechanisms of actionbehind EGCG action involves induction of terminal differen-tiation in epidermal keratinocytes and caspase-14, member ofthe caspase family associated with epithelial cell differentiation,planned cell death, and barrier formation (Hsu et al., 2007).
Katiyar et al., (2007) reviewed the most recent investigationsand elucidated that prevention of skin cancer is mediated in-volving different mechanisms and important considerations aregiven to the induction of immunoregulatory cytokine interleukin(IL) 12, IL-12-dependent DNA repair following nucleotide ex-cision repair mechanism, the inhibition of UV-induced immuno-suppression, the inhibition of angiogenic factors, and the stimu-lation of cytotoxic T cells in a tumor microenvironment (Meeranet al., 2006).
Lung Cancer
Lung cancer, also called as bronchogenic cancer, is one ofthe most common cancers in the world and caused by the rapidgrowth and division of cells that make up the lungs. Treatmentof lung cancer depends upon a variety of factors and histopatho-logic groupings into small cell lung carcinoma (SCLC) andnon-small cell lung carcinoma (NSCLC) may be used to betterpredict a patient’s response to chemotherapy.
Small-cell lung carcinoma (SCLC) has a poor prognosis,particularly due to the development of drug resistance. Sadavaet al. (2007) observed that incubation of human SCLC cells inEGCG for 24 h resulted in 50–60% reduced telomerase activityand reduction in activities of caspases 3 (50%) and 9 (70%)but caspase 8 and DNA fragmentation remain unaffected withEGCG treatment. It has the capacity to block the cell-cycle in Sphase indicating the potential use of EGCG, and possibly greentea, in treating SCLC.
Liao et al., (2004) indicated that Polyphenon E extracted fromgreen tea administration significantly reduced the incidence (by52%) and multiplicity (by 63%) of lung adenocarcinoma. Inhi-bition of angiogenesis and the induction of apoptosis by greentea may be closely related to the inhibition of pulmonary car-cinogenesis (Lu et al., 2006). (-)-Epicatechin-enhanced apop-tosis, growth inhibition of human lung cancer cell line PC-9cells, while inhibition of TNF release from BALB/c-3T3 cellsby EGCG and other tea polyphenols with a galloyl moiety in adose-dependent manner reveals their possible potential againstlung carcinoma (Suganuma et al., 1999).
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Significant evidences are there that whole green tea is a morereasonable mixture for cancer prevention in humans than EGCGalone because it is more effective when it is used in combina-tion with other cancer preventives. Mechanisms of green tea forchemoprevention of lung cancer includes antioxidant activity,phase II enzymes induction, inhibition of TNFα expression andrelease, inhibition of cell proliferation, and induction of apop-tosis. Inhibition of key protein kinases involved in cell cycleregulation and induction of apoptosis are probably the two mostsignificant effects launched by the utilization of green tea (Clarkand You, 2006; Fujiki et al., 2002; Xu et al., 1992).
Prostate Cancer
Prostate cancer is the most common male cancer in devel-oped countries and is increasing in the developing world. Itslong latency and geographical variation suggest the possibil-ity of prevention or postponement of onset by dietary supple-ments (Jian et al., 2007). A number of epidemiological studieshave suggested that consumption of green tea reduces the riskof prostate cancer. Because of unfavorable prognosis of extra-prostatic, prevention is considered the best approach to fight itat the present time (Bettuzzi et al., 2007). Adhami et al., (2004)explored the role of green tea polyphenols in modulating theIGF-1—driven molecular pathway in prostate tumor cells in amouse model and their results were quite conclusive that theIGF-I/IGFBP-3 signaling pathway is a prime pathway for greentea polyphenol–mediated inhibition of prostate cancer whichlimits the progression of cancer through inhibition of angiogen-esis and metastasis.
The treatment with tea ingredients results in (i) significantinhibition in growth of implanted prostate tumors, (ii) reductionin the level of serum prostate specific antigen, (iii) induction ofapoptosis accompanied with upregulation in Bax and decreasein Bcl-2 proteins and (iv) decrease in the level of VEGF protein(Siddique et al., 2006). EGCG inhibits COX-2 without affect-ing COX-1 expression at both the mRNA and protein levels, inandrogen-sensitive LNCaP and androgen-insensitive PC-3 hu-man prostate carcinoma cells (Paschka et al., 1998). The inhi-bition induced by EGCG was found to occur via apoptotic celldeath as shown by changes in nuclear morphology and DNAfragmentation (Hussain et al., 2005).
Based on literature scanned, it can be suggested that a con-sumption of whole green tea or a combination of EGCG withchemotherapeutic drugs could be an improved strategy for pre-vention and treatment of prostate cancer.
Breast Cancer
The most common malignancy in women worldwide is breastcancer. Several experimental studies explored that green teahas anticarcinogenic effects against breast cancer (Zhang et al.,2007; Zick et al., 2006).
GTP and its principal constituent EGCG are effective insuppressing the proliferation of MDA-MB-231, a highly inva-sive estrogen receptor-negative breast cancer cell line as shownby growth inhibition and apoptosis induction (Thangapazhamet al., 2006). Treatment of human MCF-7 cells with 50 microMEGCG can bring some positive changes as apoptosis, mito-chondrial membrane potential changes, and activation of c-JunN-terminal kinase (JNK), caspase-9 and caspase-3 (Hsuuw andChan, 2007).
In another study EGCG suppressed cell viability and inducedapoptosis by the down-regulation of telomerase and inhibitedangiogenesis by reducing the expression of vascular endothe-lial growth factor (VEGF) in a dose-dependent (Mittal et al.,2004; Sartippour et al., 2002). The rate of apoptosis and activityof caspase-3 induced by EGCG was time, and dose, depen-dent. These findings suggest that EGCG might be useful in thetreatment and/or prevention of breast cancer by inducing apop-tosis. In vitro, epigallocatechin, another major catechin in greentea, also has strong effects in inducing apoptosis and inhibitinggrowth of breast cancer cells (Stuart et al., 2007; Zhao et al.,2006; Vergote et al., 2002).
Epidemiologic studies have suggested that the regular con-sumption of tea, particularly green tea, moderately decreases therisk of cancer (Weiseburger et al., 1998). These results were allsupported with meta-analysis carried out by Sun et al., (2006)and Zhang et al., (2007) gave conclusive ideas regarding pre-vention and eliminating the risk of breast cancer.
MECHANISM OF ACTION
The studies suggested that multiple mechanisms are in-volved, including induction of apoptosis, cell cycle arrest down-regulation of telomerase, inhibition of vascular endothelialgrowth factor, and suppression of aromatase activity (Stuartet al., 2007; Mittal et al., 2004; Way et al., 2004; Sartippouret al., 2002; Liang et al., 1999).
The protective effects of green tea polyphenols have beenattributed to the inhibition of enzymes such as the cytochromesP450, which are involved in the bio-activation of carcinogens.Studies have also shown the involvement of Phase II detoxi-fication enzymes during the biological response to green tea.Because the 5′ flanking regions of Phase II genes contain anantioxidant-responsive element (ARE) which is believed to me-diate the induction of Phase II enzymes by many drugs, theinvolvement of the MAPK pathway was studied by Yu et al.,(1997) as a mechanism of biological response to green teapolyphenols. Their study provided evidence that the activationof the MAPK pathway might be due to a potential signalingpathway involved in the regulation of phase II enzyme geneexpression.
Enhancement of enzymatic pathways like glutathione per-oxidase, catalase, NADPH-quinone oxidoreductase, and glu-tathione S-transferase activities that play a role in the detox-ification of carcinogenic metabolites formation by P-450 are
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key determinants for cancer initiation and may be expected tohave protective functions against carcinogenesis (Khan et al.,1992). The green tea polyphenols stimulate the transcriptionof Phase II detoxifying enzymes through ARE. Mukhtar andAhmad, (1999) suggested that green tea polyphenol treatmentresults in significant activation of MAPK, extracellular, signal-regulated kinase-2 (ERK2), as well as JNK1 and an increasein the mRNA levels of early response genes c-jun and c-fas.Jankun et al., (1997) also highlighted the anti-cancer activity ofEGCG and associated it with the inhibition of urokinase, is oneof the most frequently expressed enzymes in human cancers.
Fujiki et al., (1998) demonstrated that EGCG and other teapolyphenols inhibit growth of human lung cancer (PC-9) cellswith a G2/M phase arrest of the cell cycle. The activation of theepidermal growth factor receptor (EGFR) tyrosine kinase by itsligand is believed to initiate multiple cellular responses associ-ated with cell proliferation. Liang et al., (1997) elucidated thatEGGG significantly inhibit both DNA synthesis and the proteintyrosine activities of EGFR, platelet-derived growth factor re-ceptor (PDGFR), and fibroblast growth factor receptor (FGFR).Pannala et al., (1997) attributed the ability of green tea polyphe-nols to (i) inhibition of OONO-mediated tyrosine-nitration, and(ii) limiting surface charge alteration of low density lipoprotein(LDL). Activation of AP-1 plays an important role in tumorpromotion (McCarty, 1998). Dong et al., (1997) investigatedthe antitumor promoting effects of EGCG and theaflavins andreported that both of these were found to inhibit EGF- or TPA-induced cell transformation, as well as AP-1-dependent tran-scriptional activity and DNA binding activity. He further foundthat the inhibition of AP-1 activation occurs via the inhibitionof a c-Jun NH2-terminal kinase (JNK)-dependent pathway. Luet al., (1998) investigated some possible mechanisms involvedwith the antiproliferation activity of EGC and demonstrated thatit reduces the level of c-jun mRNA, phosphorylated JNK1, andJNK1-kinase activities.
CONCLUSIONS
Green tea and its constituents may open up a new thera-peutic avenue because of its anti-inflammatory and antioxidantpotential. It is now considered as an important component in nu-tritional support programs to prevent lifestyle-related diseasesespecially cancer. The health benefits of green tea are often at-tributed to polyphenols especially EGCG, recognized as an ef-fective chemopreventive agent that holds the ability to scavengefree radicals and is certainly helpful against various cancers.EGCG ameliorates the overproduction of pro-inflammatory me-diators and subsequent formation of peroxynitrite. Though stud-ies on the effects of green tea on some cancer cell lines provedits effectiveness, however, some conflicting results necessitatefurther research before a conclusive approach is drawn. In con-clusion the consumption of green tea regularly or its possibleinclusion in diet-based therapies could yield certain health ben-efits and potential defense against malignancies.
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