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Cancer chemoprevention with dietary phytochemicals


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Young-Joon Surh
Chemoprevention refers to the use of agents to inhibit, reverse or retard tumorigenesis.
Numerous phytochemicals derived from edible plants have been reported to interfere with a
specific stage of the carcinogenic process. Many mechanisms have been shown to account
for the anticarcinogenic actions of dietary constituents, but attention has recently been
focused on intracellular-signalling cascades as common molecular targets for various
chemopreventive phytochemicals.

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Cancer chemoprevention with dietary phytochemicals

  1. 1. REVIEWS CANCER CHEMOPREVENTION WITH DIETARY PHYTOCHEMICALS Young-Joon Surh Chemoprevention refers to the use of agents to inhibit, reverse or retard tumorigenesis. Numerous phytochemicals derived from edible plants have been reported to interfere with a specific stage of the carcinogenic process. Many mechanisms have been shown to account for the anticarcinogenic actions of dietary constituents, but attention has recently been focused on intracellular-signalling cascades as common molecular targets for various chemopreventive phytochemicals. Cancer is a growing health problem around the world A wide array of substances derived from the diet — particularly with the steady rise in life expectancy, have been found to stimulate the development, growth increasing urbanization and the subsequent changes in and spread of tumours in experimental animals, and to environmental conditions, including lifestyle. According transform normal cells into malignant ones. These are to a recent report by the World Health Organization regarded as suspected human carcinogens. (WHO), there are now more than 10 million cases of So, many dietary constituents can increase the risk of cancer per year worldwide. In 2003, it is estimated that developing cancer, but there is also accumulating evi- approximately 1,300,000 new cases of cancer will be dence from population as well as laboratory studies to diagnosed, and more than 550,000 people will die from support an inverse relationship between regular con- cancer in the United States alone. sumption of fruit and vegetables and the risk of specific Although there is no ‘magic bullet’ that can com- cancers. Several organizations — such as the WHO, the pletely conquer cancer, many types of the disease American Cancer Society, the American Institute of might be avoidable. Cancer risk can be reduced by Cancer Research (AICR) and the National Cancer eliminating the identified carcinogens — or at least Institute (NCI) — have established dietary guidelines to minimizing exposure to them — but, without com- help people reduce the cancer risk (for further informa- plete identification of the corresponding risk factors, tion, see the 1997 World Cancer Research Fund and such primary prevention might be difficult to imple- AICR report in online links box). ment. Furthermore, the avoidance of some risk factors Many clinical trials on the use of nutritional supple- could require large lifestyle changes, which are not easy ments and modified diets to prevent cancer are ongo- to implement. ing. It is conceivable that in the future people might only It has been estimated that more than two-thirds of need to take specially formulated pills that contain sub- human cancers could be prevented through appropriate stances derived from edible plants to prevent cancer or lifestyle modification. Richard Doll and Richard Peto delay its onset2. However, a precise assessment of the have reported that 10–70% (average 35%) of human mechanisms by which the components of fruit and veg- cancer mortality is attributable to diet1. Their observa- etables prevent cancer is necessary before they can beCollege of Pharmacy, tions, which are based on statistical and epidemiological recommended for inclusion in dietary supplements orSeoul National University, data, mainly concerned dietary factors that increase risk. before they can be tested in human intervention trials.Shinlim-dong, Kwanak-ku, Although the exact percentage is uncertain, there are Phytochemicals are non-nutritive components in theSeoul 151-742, South Korea.e-mail: several lines of compelling evidence from epidemiologi- plant-based diet (‘phyto’ is from the Greek word cal, clinical and laboratory studies that link cancer risk ing plant) that possess substantial anticarcinogenic anddoi:10.1038/nrc1189 to the nutritional factors. antimutagenic properties. Given the great structural768 | OCTOBER 2003 | VOLUME 3
  2. 2. REVIEWS Summary diversity of phytochemicals, it is not feasible to define structure–activity relationships to deduce their • Many population-based studies have highlighted the ability of macronutrients and underlying molecular mechanisms. A better approach micronutrients in vegetables and fruit to reduce the risk of cancer. Recently, attention is to analyse their effects on cancer-associated has been focused on phytochemicals — non-nutritive components in the plant-based signal-transduction pathways. diet that possess cancer-preventive properties. • Despite remarkable progress in our understanding of the carcinogenic process, Importance of plant-derived foods the mechanisms of action of most chemopreventive phytochemicals have not been More than 250 population-based studies, including fully elucidated. case–control and cohort studies, indicate that people • Chemopreventive phytochemicals can block initiation or reverse the promotion stage who eat about five servings of fruit and vegetables a day of multistep carcinogenesis. They can also halt or retard the progression of have approximately half the risk of developing cancer — precancerous cells into malignant ones. particularly cancers of the digestive and respiratory • Many molecular alterations associated with carcinogenesis occur in cell-signalling tracts — of those who eat fewer than two servings. In pathways that regulate cell proliferation and differentiation. One of the central the United States, these observations led to the develop- components of the intracellular-signalling network that maintains homeostasis is the ment of public-health campaigns such as the ‘Five-a-Day family of mitogen-activated protein kinases (MAPKs). for Better Health’ programme and a more recent ‘Savor • Numerous intracellular signal-transduction pathways converge with the activation the Spectrum’ campaign — both were designed to of the transcription factors NF-κB and AP1. As these factors mediate pleiotropic increase the ingestion of fruit and vegetables by the effects of both external and internal stimuli in the cellular-signalling cascades, they population (BOX 1). Increased consumption of fruit and are prime targets of diverse classes of chemopreventive phytochemicals. vegetables is a global priority in the prevention of cancer • Basic helix–loop–helix transcription factors such as NRF2 regulate expression of phase II and other chronic disorders. According to the WHO enzymes, which detoxify carcinogens and protect against oxidative stress. A number of Report 2002, there are at least 2.7 million deaths globally phytochemicals have been shown to induce expression of phase II enzymes via NRF2. per year, which are primarily attributable to low fruit • β-Catenin, a multifunctional protein that was originally identified as a component and vegetable intake. of cell–cell adhesion machinery, is another important molecular target for Vegetables and fruit are excellent sources of cancer- chemoprevention. Several dietary phytochemicals have been shown to target preventive substances. The NCI has identified about this molecule. 35 plant-based foods that possess cancer-preventive Box 1 | Chemoprevention initiatives A number of government programmes have been created in the United States and in Europe to increase vegetable consumption and decrease cancer incidence. These include the following: The ‘Five-A-Day for Better Health’ programme Founded in late 1991, this is the first nationwide health-promotion campaign to encourage people in the United States to eat fruit and vegetables — at least five servings a day — to reduce the risk of cancer and other chronic diseases. Over the past decade, there has been a steady increase in both awareness of the health benefits of fruit and vegetables and their consumption in the United States. The National Cancer Institute (NCI) has recently completed a review of this programme and reported a series of recommendations for the next round of the initiative (for further information, see the ‘Five-A-Day for Better Health’ report in online links box). ‘Savor the Spectrum’ The NCI’s spring 2002 media promotion, entitled ‘Savor the Spectrum’, urges all Americans to eat five to nine servings of colourful fruit and vegetables a day for better health. The message of this programme is based on current research showing that phytonutrients from different colour groups are powerful disease fighters that help our body fight off cancer and heart disease. NCI has produced a series of guidelines featuring each colour of the‘rainbow’ of fruit and vegetables. European Prospective Investigation of Cancer and Nutrition (EPIC) EPIC is one of the most important multicentre prospective cohort studies ever launched worldwide. Beginning in 1992, EPIC has involved more than half a million (520,000) participants recruited by 20 centres in 10 countries under the coordination of the International Agency for Research on Cancer (IARC) and partly funded by the ‘Europe Against Cancer’ programme of the European Commission, as well as by the participating countries. EPIC focuses on identifying the dietary determinants of cancer, and is aimed at expanding the presently limited knowledge of the role of nutrition and other lifestyle factors in the aetiology and prevention of cancer and other life-threatening diseases. Global Strategy on Dietary Prevention of Cancer For a global extension of the ‘Five-A-Day’ concept of boosting increased consumption of fruit and vegetables, the WHO organized the third Biennial ‘Five-A-Day’ International Symposium on January 14–15 2003 in Berlin, Germany. At the meeting, Derek Yach, the WHO Executive Director of Noncommunicable Diseases & Mental Health, said “Increasing the consumption of fruit and vegetables is a necessary part of the effort to reduce the growing global burden of chronic diseases including cancer.” The guidelines stated “choose most of the foods you eat from plant sources”.NATURE REVIEWS | C ANCER VOLUME 3 | OCTOBER 2003 | 7 6 9
  3. 3. REVIEWS Pro-carcinogen cancer. Recently, the focus and emphasis have shifted to Detoxification Secretion the non-nutritive phytochemicals. The NCI has deter- mined in laboratory studies that more than 1,000 differ- Metabolic ent phytochemicals possess cancer-preventive activity. It activation is estimated that there could be more than 100 different Cancer-blocking phytochemicals in just a single serving of vegetables. agents Detoxification As early as 1980, the NCI’s Chemoprevention Ellagic acid Indole-3-carbinol Ultimate Programme of the Division of Cancer Prevention and carcinogen Sulphoraphane Control began evaluating phytochemicals for safety, effi- Flavonoids cacy and applicability for cancer prevention. Michael Sporn coined the term ‘chemoprevention’ in the mid- 1970s to describe the strategy of blocking or slowing the Normal onset of premalignant tumours with relatively nontoxic cell chemical substances. To better define and guide research in the field of chemoprevention, the NCI Division of Initiation Preneoplastic Neoplastic Cancer Prevention started the Chemoprevention (1–2 days) cells cells Implementation Group in 1998, and then the Rapid Promotion Progression Access to Preventive Intervention Development pro- (>10 years) (>1 year) gramme. The NCI has more than 400 potential agents under investigation and is sponsoring more than 65 Initiated cell Phase I, Phase II and Phase III chemoprevention trials. These involve various substances or their mixtures, Cancer-suppressing many of which are foodborne phytochemicals. agents β-Carotene Curcumin Mechanisms of chemoprevention EGCG Carcinogenesis is generally recognized as a multistep Genistein Resveratrol process in which distinct molecular and cellular alter- [6]-Gingerol ations occur. From the study of experimentally induced Capsaicin carcinogenesis in rodents, tumour development is con-Figure 1 | Dietary phytochemicals that block or suppress multistage carcinogenesis. sidered to consist of several separate, but closely linked,Carcinogenesis is initiated with the transformation of the normal cell into a cancer cell (initiated stages — tumour initiation, promotion and progression.cell). These cells undergo tumour promotion into preneoplastic cells, which progress to neoplasticcells. Phytochemicals can interfere with different steps of this process. Some chemopreventive Although these divisions are an oversimplification ofphytochemicals inhibit metabolic activation of the procarcinogens to their ultimate electrophilic carcinogenesis, it is useful to think in these stages whenspecies, or their subsequent interaction with DNA. These agents therefore block tumour initiation considering possible opportunities for chemoprevention.(blocking agents). Alternatively, dietary blocking agents can stimulate the detoxification of Initiation is a rapid and irreversible process thatcarcinogens, leading to their secretion from the body. Other phytochemicals suppress the later involves a chain of extracellular and intracellularsteps (promotion and progression) of multistage carcinogenesis (suppressing agents). Some events. These include the initial uptake of or exposurephytochemicals can act as both blocking and suppressing agents. Adapted from REF. 128. to a carcinogenic agent, its distribution and transport to organs and tissues where metabolic activation and detoxification can occur, and the covalent interaction of properties. These include garlic, soybeans, ginger, reactive species with target-cell DNA, leading to geno- onion, turmeric, tomatoes and cruciferous vegetables toxic damage. In contrast to initiation, tumour promo- (for example, broccoli, cabbage, cauliflower and tion is considered to be a relatively lengthy and Brussels sprouts). Numerous cell-culture and animal- reversible process in which actively proliferating pre- model studies have been conducted to evaluate the neoplastic cells accumulate. Progression, the final stage ability of specific edible plants to prevent cancer. of neoplastic transformation, involves the growth of a tumour with invasive and metastatic potential. Beyond vitamins to phytochemicals According to the conventional classification origi- Many population-based studies have highlighted the nally proposed by Lee Wattenberg, chemopreventive ability of macronutrients (for example, carbohydrate, agents are subdivided into two main categories — proteins, fat and fibre) and micronutrients (for exam- blocking agents and suppressing agents3. Blocking ple, antioxidant vitamins and trace minerals) that are agents prevent carcinogens from reaching the target contained in vegetables and fruit to reduce the risk of sites, from undergoing metabolic activation or from cancer. The most exciting findings have been achieved subsequently interacting with crucial cellular macro- with antioxidant vitamins and their precursors, which molecules (for example, DNA, RNA and proteins). are found in dark, leafy green vegetables and Suppressing agents, on the other hand, inhibit yellow/orange fruit and vegetables. The NCI has there- the malignant transformation of initiated cells, in fore sponsored a series of human intervention trials either the promotion or the progression stage. with individual vitamins and minerals. However, plants Chemopreventive phytochemicals can block or reverse contain numerous chemical substances other than these the premalignant stage (initiation and promotion) of micronutrients that might also be useful in preventing multistep carcinogenesis. They can also halt or at least770 | OCTOBER 2003 | VOLUME 3
  4. 4. REVIEWS Turmeric Grapes OH O O O O H3C CH3 HO HO OH OH Curcumin Resveratrol Chilli peppers Honey O O O HO H3C N O H HO HO Capsaicin Caffeic acid phenethyl ester Ginger Garlic O OH O H3C S HO [6]-Gingerol Diallyl sulphide HO OH Green tea Cabbage OH OH O OH O N HO OH H O Indole-3-carbinol OH OH Epigallocatechin-3-gallate Soybeans Broccoli OH OH O S C H3C N S HO O O Genistein Sulphoraphane Tomatoes Lycopene Figure 2 | Representative chemopreventive phytochemicals and their dietary sources. retard the development and progression of precancer- include carcinogen activation/detoxification by xenobi- ous cells into malignant ones (FIG. 1). Recent advances otic metabolizing enzymes; DNA repair; cell-cycle in our understanding of the carcinogenic process at progression; cell proliferation, differentiation and apop- the cellular and molecular level have shown this block- tosis; expression and functional activation of oncogenes ing and suppressing categorization to be an oversim- or tumour-suppressor genes; angiogenesis and metasta- plification, and numerous cellular molecules and sis; and hormonal and growth-factor activity (for events that could be potential targets of chemopreven- further information, see ONLINE TABLE 1). tive agents have been more specifically identified4–6. Therefore, the ability of any single chemopreventive Cellular signalling molecules as targets phytochemical to prevent tumour development During the past two or three decades, there has been should be recognized as the outcome of the combina- substantial progress in identifying the biochemical tion of several distinct sets of intracellular effects, events that are associated with the multistage process rather than a single biological response. of carcinogenesis, and we are now better aware of how FIGURE 2 illustrates the chemical structures of repre- certain dietary phytochemicals are able to alter this sentative dietary phytochemicals that have been known process (FIG. 1). Remarkable advances in the cellular to possess chemopreventive potential and their dietary and molecular genetics of carcinogenesis — such as sources. The cellular and molecular events affected or the identification of numerous oncogenes and regulated by these chemopreventive phytochemicals tumour-suppressor genes, specific genes encodingNATURE REVIEWS | C ANCER VOLUME 3 | OCTOBER 2003 | 7 7 1
  5. 5. REVIEWS Despite this progress, the identification of molecular and cellular targets of chemopreventive phytochemicals is still incomplete. Many of the molecular alterations that PI3K MEKK1 RAS PKC Curcumin EGCG are associated with carcinogenesis occur in cell-signalling Resveratrol pathways that regulate cell proliferation and differentia- tion. One of the central components of the intracellular- EGCG signalling network that maintains homeostasis is the PDK NIK MEK1/2 RAF family of proline-directed serine/threonine kinases — Cytoplasm MKK4 the mitogen-activated protein kinases (MAPKs; FIG. 3). Abnormal or improper activation or silencing of the MAPK pathway or its downstream transcription fac- Genistein tors can result in uncontrolled cell growth, leading to EGCG AKT IKK-α/β/γ ERK1/2 p38 JNK Curcumin malignant transformation. Some phytochemicals EGCG Curcumin Resveratrol ‘switch on’ or ‘turn off ’ the specific signalling mole- EGCG cule(s), depending on the nature of the signalling cas- Genistein cade they target, preventing abnormal cell proliferation Resveratrol Capsaicin and growth4–12. Cell-signalling kinases other than MAPKs, such as protein kinase C (PKC) and phos- phatidylinositol 3-kinase (PI3K), are also important ELK1/SAP1 SRF c-JUN ATF2 targets of certain chemopreventive phytochemicals. P These upstream kinases activate a distinct set of tran- c-FOS c-JUN SRE TRE NF-κB IκB NF-κB scription factors, including nuclear factor κB (NF-κB) and activator protein 1 (AP1; FIG. 3). Ub NF-κB and AP1 AP1 c-FOS c-JUN Numerous intracellular signal-transduction pathways NF-κB 26S converge with the activation of the transcription factors Curcumin κB binding site TRE NF-κB and AP1, which act independently or coordinately to regulate target-gene expression (FIG. 3). Nucleus Aberrant activation of NF-κB has been associated Proteasome with protection against apoptosis and stimulation of proliferation in malignant cells13,14, and overexpressionFigure 3 | Effect of phytochemicals on activation of NF-κB and AP1. The NF-κB signallingpathway converges on the multiprotein complex called the IκB kinase (IKK) signalsome, of NF-κB is causally linked to the phenotypic changesleading to IκB phosphorylation (P), ubiquitylation (Ub) and subsequent degradation by the 26S that are characteristic of neoplastic transformation15.proteasome. NF-κB is then released and translocated to the nucleus, where it binds to specific Many chemopreventive phytochemicals that arepromoter regions of various genes. The IKK signalsome is activated by the NF-κB-inducing derived from the diet have been shown to suppresskinase (NIK). Pathways that regulate NIK are likely to involve signalling through a family of constitutive NF-κB activation in malignant cells ormitogen-activated protein kinases (MAPKs), such as MAPK kinase kinase-1 (MEKK1) — a NF-κB activation induced by the external tumour pro-kinase that lies upstream of extracellular signal-regulated kinase (ERK) — MAPK/ERK kinase(MEK1/2) and p38 MAPK. Recent reports showed that NF-κB activation is also regulated by moter phorbol 12-myristate 13-acetate (PMA) orthe AKT signalling pathway58,59,129. Phosphatidylinositol 3-kinase (PI3K) activates AKT/protein tumour-necrosis factor-α (TNF-α)11,16,17.kinase B via phosphorylation by 3-phosphoinositide-dependent protein kinase-1 (PDK1). AP1 is another transcription factor that regulatesGenistein specifically inhibits AKT activity and AKT-mediated NF-κB activation58,59. expression of genes that are involved in cellular adapta-Epigallocatechin gallate (EGCG) can block the activities of PI3K and AKT49. There is crosstalk tion, differentiation and proliferation. Functional activa-between the AKT and NF-κB signalling pathways — AKT phosphorylation leads to activation of tion of AP1 is associated with malignant transformationNF-κB by stimulating IκB kinase (IKK) activity129. IKK is also a target for chemopreventive as well as tumour promotion18–21. AP1 consists of eitherphytochemicals, including curcumin24,28, resveratrol71 and EGCG45,130. The MAPK familyproteins also regulate expression of AP1 — a heterogenous set of dimeric proteins made up of homo- or heterodimers between members of the JUNmembers of the c-JUN, c-FOS and ATF families. In this pathway, activation of ERK1/2 and FOS families, which interact via a leucine-zipperphosphorylates ELK1, c-JUN NH2-terminal kinase (JNK) phosphorylates c-JUN, and p38 domain. This transcription factor is also regulated byphosphorylates both ELK1 and ATF2. This leads to transcriptional activation of target genes. the MAPK-signalling cascade21–23.External stimuli — including phorbol ester and ultraviolet radiation — activate specific isoforms As NF-κB and AP1 are ubiquitous eukaryotic tran-of protein kinase C (PKC), which, in turn, leads to stimulation of the p21 RAS–ERK signalling scription factors that mediate pleiotropic effects of bothpathway via RAF and MEK1/2. Activation of p38 and JNK is mediated by MAPK kinase-4(MKK4), which is under control of the upstream kinase MEKK. external and internal stimuli in the cellular-signalling cascades, they are prime targets of diverse classes of chemopreventive phytochemicals (FIG. 3). carcinogen-metabolizing enzymes, DNA-repair Phytochemicals targeting NF-κB and AP1 enzymes and proteins, and regulators of cell cycle and Curcumin, [6]-gingerol and capsaicin. Curcumin — a apoptosis — have given us a better insight into the yellow pigment that is present in the rhizome of process of neoplastic transformation. Advances have turmeric (Curcuma longa L.) and related species — is also been made in identifying the factors that mediate one of the most extensively investigated phytochemicals, tumour invasion, metastasis and angiogenesis. with regard to chemopreventive potential. Curcumin772 | OCTOBER 2003 | VOLUME 3
  6. 6. REVIEWS has been shown to suppress tumour promotion in a activation of JNK and p38, and deactivation of ERK41. mouse model of skin carcinogenesis. Furthermore, pre- Pharmacological inhibition or dominant-negative treatment of human colonic epithelial cells with cur- forms of JNK and p38, but not of ERK, abrogated the cumin inhibited TNF-α-induced cyclooxygenase-2 capsaicin-induced apoptosis in these cells41. (COX2) gene transcription and NF-κB activation24. In this study, curcumin inhibited IκB degradation by Epigallocatechin gallate (EGCG). EGCG is an antioxi- downregulation of NF-κB-inducing kinase (NIK) and dant and chemopreventive polyphenol that is found in IκB kinase (IKK)α/β. green tea. It has been shown to suppress malignant When curcumin was applied topically to the dorsal transformation in a PMA-stimulated mouse epidermal skin of female ICR mice (a model initially developed at JB6 cell line, which seemed to be mediated by blocking the Institute of Cancer Research, Fox Chase Cancer activation of Ap1 (REFS 42,43) or Nf-κb44. More recently, Center), it prevented the PMA-induced activation of EGCG treatment of human epidermal keratinocytes both Nf-κb and Ap1 (REF. 25). The inhibition of Nf-κb resulted in significant inhibition of ultraviolet (UV)- was accompanied by blockade of degradation via phos- B-light-induced activation of IKKα, phosphorylation phorylation of Iκbα and also by reduced nuclear translo- and subsequent degradation of IκBα and nuclear cation of the p65 subunit of Nf-κb (REF. 26; FIG. 3). translocation of p65 (REF. 45). In the Hras-transformed Topically applied curcumin inhibited the catalytic activ- epidermal JB6 cells, EGCG inhibited Ras-activated Ap1 ity of epidermal extracellular-signal-regulated kinase activity 46,47. Similar Ap1 inhibition was observed in the (Erk)1/2, which could account for its ability to inactivate epidermis of transgenic mice that harbour an Ap1-driven Nf-κb and Cox2 (REF. 26). Curcumin also suppressed the luciferase reporter gene. TNF-α-induced nuclear translocation and DNA binding Nomura and colleagues48 have reported the of NF-κB in a human myeloid leukaemia cell line by inhibitory effect of EGCG on UV-light-induced PI3K blocking phosphorylation and subsequent degradation activation in mouse epidermal cells. The reduction of of IκB27. PMA- and hydrogen-peroxide-induced activa- signalling via PI3K–AKT–NF-κB by EGCG was tion of NF-κB was similarly attenuated by curcumin reported to be mediated through inhibition of ERBB2 treatment. In addition, curcumin inhibited IκBα phos- (also known as HER2/NEU) receptor tyrosine phos- phorylation in human multiple myeolma cells28 and phorylation49. EGCG also inhibited vascular endothelial murine melanoma cells29 through suppression of IKK growth factor (VEGF) production by inhibiting consti- activity, which contributed to its antiproliferative, tutive activation of both STAT3 and NF-κB — but not proapoptotic and/or antimetastatic activities. of ERK or AKT — in human breast and head and neck [6]-Gingerol — a phenolic substance that is responsi- cancer cell lines50. ble for the spicy taste of ginger (Zingiber officinale EGCG treatment resulted in inhibition of cell Roscoe) — was reported to inhibit tumour promotion growth, G0/G1-phase arrest of the cell cycle and induc- and PMA-induced ornithine decarboxylase (ODC) activ- tion of apoptosis in human epidermoid carcinoma ity and Tnf-α production in mouse skin30. More recently, (A431) cells, but not in normal human epidermal ker- [6]-gingerol has been found to inhibit epidermal growth atinocytes (NHEK)51. A431 cells were more susceptible factor (Egf)-induced Ap1 activation and neoplastic trans- to EGCG-mediated inhibition of constitutive NF-κB formation in mouse epidermal JB6 cells — this was expression and activation than NHEK cells, indicating shown using reduced anchorage-independent formation that EGCG-caused cell-cycle deregulation and apoptosis of cell colonies in soft agar 31. of cancer cells might be mediated through NF-κB inhi- Capsaicin — a pungent component of hot chilli pep- bition. The roles of EGCG and other tea polyphenols on per (Capsicum annuum L.) — has been suspected to act cellular signalling have been reviewed recently 52,53. as a carcinogen or a co-carcinogen in experimental ani- mals because of its irritant properties, but other studies Genistein. Genistein — a soy-derived isoflavone — is indicate that the compound has chemopreventive and believed to contribute to the putative breast- and chemoprotective effects32–35. Topical application of cap- prostate-cancer-preventive activity of soya. Genistein saicin inhibited PMA-induced mouse-skin tumour for- inhibited PMA-induced AP1 activity, expression of mation36 and activation of Nf-κb37. This was attributed c-FOS and ERK activity in certain human mammary to blockade of Iκbα degradation and Nf-κb transloca- cell lines54. Genistein treatment abrogated NF-κB DNA tion into the nucleus. PMA- or Tnf-α-induced Ap1 acti- binding in human hepatocarcinoma cells stimulated vation in mouse skin and cultured human leukaemia with hepatocyte growth factor55. The downregulation HL-60 cells was also blocked by capsaicin38. of c-Jun and c-Fos by genistein was also observed in Capsaicin inhibited constitutive and induced acti- UV-light-stimulated skin of SENCAR (sensitivity to vation of NF-κB in human malignant-melanoma cells, carcinogenesis) mice56. leading to inhibition of melanoma-cell proliferation39. Genistein at the apoptogenic concentration also Capsaicin also induced apoptosis in cultured Jurkat inhibited the H2O2- or TNF-α-induced activation of cells through generation of reactive oxygen species NF-κB in both the androgen-sensitive (LNCaP) and - (ROS) and rapid activation of c-JUN NH2-terminal insensitive (PC3) human prostate cancer cell lines by kinase (JNK)40. Similarly, capsaicin caused apoptotic reducing phosphorylation of IκBα and the nuclear death in HRAS-transformed human mammary translocation of NF-κB57. Genistein-mediated inactivation epithelial cells, which was accompanied by marked of NF-κB was associated with downregulation of AKT inNATURE REVIEWS | C ANCER VOLUME 3 | OCTOBER 2003 | 7 7 3
  7. 7. REVIEWS the prostate cancer58 and mammary cancer59 cells. The HeLa cell cultures, which was associated with inhibition same studies also revealed that AKT transfection led to of PKC and protein tyrosine kinase68. Similarly, resvera- the activation of NF-κB, which was completely blocked trol blocked UV-light-induced activation of NF-κB by genistein treatment, indicating that inhibition of the through suppression of IKK activation69. Resveratrol crosstalk between AKT and NF-κB could provide a suppressed TNF-α-induced phosphorylation and novel mechanism responsible for pro-apoptotic activity nuclear translocation of p65, and NF-κB-dependent of genistein. reporter-gene transcription in myeloid leukaemia PMA- or TNF-α-induced NF-κB DNA binding cells70. The suppression of NF-κB coincided with sup- and NF-κB-derived COX2 promoter activity, as well as pression of AP1. Resveratrol also inhibited the TNF- COX2 expression, were inhibited in human alveolar induced activation of MAPK kinase (MEK) and JNK, epithelial carcinoma cells by genistein treatment60. In and abrogated TNF-induced caspase activation70. human U937 monocytes, genistein exerted no sub- Resveratrol induced apoptosis in fibroblasts after the stantial inhibitory effect on DNA binding of NF-κB, induced expression of oncogenic HRAS, possiblyLUCIFERASE-REPORTER-GENEASSAY but markedly attenuated its transcriptional activity 61. through inhibition of NF-κB activation by blockingA recombinant method that is Consistent with this notion, genistein strongly sup- IKK activity71.used to measure transcriptional presses NF-κB transcriptional activity in PMA-stimu-activity in which the regulatory lated human mammary epithelial cells, as determined Miscellaneous phytochemicals. In addition to thesequence (for example, by the LUCIFERASE-REPORTER-GENE ASSAY but does not inter- aforementioned phytochemicals, caffeic acidpromoter or enhancer) ofinterest is joined to a firefly fere with IκB degradation, and subsequent nuclear phenethyl ester (CAPE), sulphoraphane, silymarin,luciferase gene that, following translocation and DNA binding of NF-κB (M.-H. apigenin, emodin, quercetin and anethole have alsoactivation, produces light from Chung and Y.-J.S., unpublished observations). been reported to suppress the activation of NF-κB andluciferin in the presence of ATP Genistein might block the phosphorylation of p65 AP1, which might contribute to their chemopreventiveadded to the assay mixture. Therelative intensity of the light without influencing the IKK activity, thereby hamper- and/or cytostatic effects16.emission is measured with a ing its interaction with co-activators such as cyclicluminometer. AMP response element binding protein (CREB)-binding NRF–KEAP1 complex protein (CBP/p300), a key element of the transcrip- Other than suppressing tumour promotion or progres-CREB tion-initiation complex that bridges DNA-bound sion, another important approach to chemoprevention(Cyclic AMP response elementbinding protein). CREB is a transcription factors to the transcription machinery. is to block the DNA damage caused by carcinogenicleucine zipper transcription insult — the initiation stage of carcinogenesis. Toxicfactor that binds to DNA at the Resveratrol. Resveratrol (3,4′,5-trihydroxy-trans- xenobiotic (‘xeno’, from the Greek word meaning ‘for-cyclic AMP response element stilbene) is a phytoalexin that is present in grapes (Vitis eign’) chemicals, including carcinogens, are detoxified(CRE) as a homo- orheterodimer. It has pivotal roles vinifera) and a key antioxidant ingredient of red wine. It by PHASE II ENZYMES — such as glutathione S-transferasein the control of cellular is believed to be responsible for the so-called ‘French (GST) and NAD(P)H:quinone oxidoreductase (NQO).proliferation and differentiation, paradox’, in which consumption of red wine has been The phase II enzyme induction system is anapoptosis, intermediary shown to reduce the mortality rates from cardiovascular important component of the cellular stress responsemetabolism, inflammation and diseases and certain cancers. Resveratrol treatment in which a diverse array of electrophilic and oxidativenumerous other responses,particularly in hepatocytes, inhibited PMA-induced COX2 expression and catalytic toxicants can be removed from the cell before theyadipocytes and haematopoietic activity, via the cyclic-AMP response element (CRE), in are able to damage the DNA. Antioxidants exert theircells. human mammary epithelial cells62,63. It also inhibited protective effects not only by scavenging ROS, but PKC activation, AP1 transcriptional activity and the also by inducing de novo expression of genes thatPHASE II ENZYMESA group of xenobiotic induction of COX2-promoter activity in PMA-treated encode detoxifying/defensive proteins, includingmetabolizing enzymes that are cells. Resveratrol induced apoptosis and reduced the phase II enzymes. Many xenobiotics activatemainly involved in the constitutive activation of NF-κB in both rat and human stress-response genes in a manner similar to thatinactivation and excretion of pancreatic carcinoma cell lines64. Mammary tumours achieved by antioxidants. These genes encodecarcinogens and other toxic isolated from rats treated with resveratrol displayed enzymes such as glutathione peroxidase, gamma-glu-chemical substances. reduced expression of Cox2 and matrix metallopro- tamylcysteine synthetase (γ-GCS), GST, NQO andANTIOXIDANT-RESPONSIVE teinase (Mmp)-9, as well as reduced Nf-κb activation, heme oxygenase-1 (HO-1). The 5′-flanking regionsELEMENT compared with controls65. Treatment of human breast of these genes contain a common cis-element, known(ARE). A specific DNA- cancer MCF-7 cells with resveratrol also suppressed as the ANTIOXIDANT-RESPONSIVE ELEMENT (ARE) (FIG. 4).promoter-binding region thatcan be transcriptionally NF-κB activation and proliferation65. Many basic leucine zipper (bZIP) transcriptionactivated by numerous Treatment of androgen-sensitive prostate cancer factors — including NRF, JUN, FOS, FRA, MAF andantioxidants and/or cells (LNCaP) with resveratrol caused downregulation AH receptor — bind to these ARE sequences andelectrophiles. Many stress- of prostate-specific antigen and p65; these effects were modulate expression of some of the aforementionedresponse genes encoding phase associated with activation of p53, WAF1, p300/CBP stress-response genes72 (FIG. 4).II detoxification or antioxidantenzymes such as glutathione and APAF1 (REF. 66). Resveratrol-induced apoptosis inS-transferase, quinone reductase, mouse JB6 epidermal cells was associated with phos- NRF. During oxidative stress or other types of toxicand heme oxygenase-1 — which phorylation of p53, which seemed to be mediated insult that are induced by xenobiotic chemicals, cer-provide defence against cellular through activation of Erk and p38 (REF. 67). Yu and col- tain members of the helix–loop–helix bZIP family ofoxidative stress — have thiselement in their 5′-flanking leagues have shown that resveratrol pretreatment gives transcription factors — particularly the nuclear fac-region to facilitate the rise to suppression of PMA- and UV-light-induced tor-erythroid 2p45 (NF-E2)-related factors (NRF1transcription process. activation of AP1 and MAPKs (ERK2, JNK and p38) in and NRF2) — heterodimerize and bind to the ARE774 | OCTOBER 2003 | VOLUME 3
  8. 8. REVIEWSCell membrane carcinogen benzo[a]pyrene, which was not prevented by oltipraz, a chemopreventive agent with phase II enzyme inducing activity75,84. Nrf2-null mice also have PI3K PKC JNK ERK Curcumin CAPE defects in detoxifying carcinogens such as aflatoxin B185. Sulphoraphane Stable transfection of L929 cells with a dominant- p38 negative mutant form of Nrf2 abolished induction of P P SR SH Ho-1 by several toxicants86. Fibroblasts from Nrf2-null S T mice were found to express only about 15% as much KEAP1 KEAP1 S T NRF2 Gcs mRNA as wild-type cells87. Overexpression of NRF2 NRF2 activated ARE-mediated transcription in human C/EBPβ hepatoma (HepG2) cells, and this activation was 6-HITC further increased by tert-butylhydroquinone88. Sulphoraphane P P KEAP1 — a negative regulator of NRF. A cytosolic S T Phase II enzymes: actin-binding protein called Kelch-like ECH-associated NRF2 GSTA-2 protein 1 (KEAP1) has been identified as a docking site NQO-1 MAF r-GCLC at which the bZIP proteins are sequestered under nor- C/EBPβ r-GCLM mal physiological conditions. For example, KEAP1 CCAAT/XRE ARE HO-1 suppresses the transcriptional activity of NRF2 by retaining the transcription factor in the cytoplasm and hampering its nuclear translocation (FIG. 4). The mechanisms by which cells recognize chemo- preventive antioxidants or phase II enzyme inducersFigure 4 | Transcriptional activation by NRF2. NRF2 is a transcription factor that regulates have not been fully elucidated. The KEAP1–NRF2expression of many detoxification or antioxidant enzymes. The Kelch-like-ECH-associated complex is an intracellular sensor that recognizesprotein 1 (KEAP1) is a cytoplasmic repressor of NRF2 that inhibits its ability to translocate to redox signalling by detecting electrophiles or ROS89.the nucleus. These two proteins interact with each other through the double glycine-rich Many phase II gene inducers are able to generate ROS,domains of KEAP1 and a hydrophilic region in the NEH2 domain of NRF2. KEAP1 containsmany cysteine residues. Phase II enzyme inducers and/or prooxidants can cause oxidation or else can be readily converted — nonenzymatically,or covalent modification (R) of these cysteine residues91. As a result, NRF2 is released from via REDOX CYCLING — or metabolized to electrophilicKEAP1. In addition, phosphorylation of NRF2 at serine (S) and threonine (T) residues by intermediates in the body. Phase II enzyme inducerskinases such as phosphatidylinositol 3-kinase (PI3K), protein kinase C (PKC)131, c-Jun NH2- mimic pro-oxidants and electrophiles, although mostterminal kinase (JNK) and extracellular-signal-regulated kinase (ERK) is assumed to facilitate of them are antioxidants by nature. Therefore, it mightthe dissociation of NRF2 from KEAP1 and subsequent translocation to the nucleus. p38 can be more appropriate to call ARE an ‘electrophileboth stimulate and inhibit the NRF2 nuclear translocation. In the nucleus, NRF2 associates response element’ (EpRE). It is plausible that thesewith small MAF (the term is derived from musculoaponeurotic-fibrosarcoma virus), forming aheterodimer that binds to the antioxidant-responsive element (ARE) to stimulate gene reactive species interact with thiol groups of KEAP1expression. NRF2/MAF target genes encode phase II detoxification or antioxidant enzymes and oxidize or covalently modify the cysteine residuessuch as glutathione S-transferase α2 (GSTA2), NAD(P)H:quinone oxidoreductase (NQO1), within KEAP1 and also, possibly, NRF2 (REFS 90–93).γ-glutamate cysteine ligase (γ -GCLC and γ -GCLM) and heme oxygenase-1 (HO-1). PI3K This would cause KEAP1 to release NRF2, so it couldalso phosphorylates the CCAAT/enhancer binding protein-β (C/EBPβ), inducing its translocate to the nucleus and activate transcription oftranslocation to the nucleus and binding to the CCAAT sequence of C/EBP-β response phase II enzymes (FIG. 4).element within the xenobiotic response element (XRE), in conjunction with NRF2 binding toARE132. Transfection of human neuroblastoma cells with PI3K activates ARE, which is In accordance with this model, sulphydryl-reactiveattenuated by a pharmacological inhibitor of PI3K or dominant-negative NRF2 (REF. 133). agents — such as diethyl maleate —abrogatedCurcumin and caffeic acid phenethyl ester (CAPE) disrupt the NRF2–KEAP1 complex, KEAP1 repression of NRF2, allowing release of theleading to increased NRF2 binding to ARE99,100. Sulphoraphane directly interacts with KEAP1 transcription factor 89. In this context, the cysteineby covalent binding to its thiol groups91. 6-(Methylsulfinyl)hexyl isothiocyanate (6-HITC) — a residues in KEAP1 could serve as a molecular sensorsulphoraphane analogue from Japanese horseradish wasabi — stimulates nuclear of intracellular redox status, ensuring the propertranslocation of NRF2, which subsequently activates ARE 98. and timely expression of genes that are involved in cellular antioxidant defence or detoxification of electrophilic toxicants. sequence to activate transcription 73. In human hepatoma cells that are genetically engineered to Phytochemicals that activate NRF overexpress NRF1 or NRF2, both basal and inducible Exposure of HepG2 cells to the green-tea extract transcriptional activities of an ARE reporter gene induces expression of phase II detoxifying enzymes were increased. through ARE94. This upregulation was accompanied A role for NRF2 in the regulation of ARE-mediated by activation of ERK2 and JNK1, as well as immediate-REDOX CYCLING gene expression has been shown in studies involving early genes c-JUN and c-FOS. Subsequent studies haveA reciprocal transformation Nrf2-null mice73. These mice fail to induce many of the shown that EGCG transcriptionally activated thebetween an oxidant and its genes involved in carcinogen detoxification and protec- phase II enzyme gene expression in HepG2 cells, asreductive counterpart. Anexample is conversion of tion against oxidative stress73–83. Most notably, the Nrf2- determined by the ARE reporter-gene assay95. In thiscatechol to quinone via null mice developed a larger number of tumours in the experiment, EGCG strongly activated all three MAPKssemiquinone or vice versa. forestomach after treatment with the ubiquitous (ERK, JNK and p38) and induced caspase-3-mediatedNATURE REVIEWS | C ANCER VOLUME 3 | OCTOBER 2003 | 7 7 5
  9. 9. REVIEWS cell death. Other phytochemicals such as phenethyl (CKI) has been shown to convert β-catenin into a isothiocyanate and sulphoraphane also differentially form that is favoured for phosphorylation by GSK-3β, regulated the activation of MAPKs and NRF, ARE- and so promotes destabilization of β-catenin107,108. Liu mediated luciferase reporter-gene activity, and phase II et al. reported a similar function of another isoform of enzyme gene induction96,97. casein kinase, CKIα109. Analysis of gene-expression profiles by an oligonu- So, β-catenin needs to be stabilized in the cyto- cleotide microarray revealed that sulphoraphane plasm to escape the degradation pathway. This occurs upregulated expression of Nqo1, Gst and Gcs in the in response to WNT signalling, as well as signalling by small intestine of wild-type mice, whereas the Nrf2-null several growth factors, such as platelet-derived mice displayed much lower levels of these enzymes80. endothelial factor and bacterial lipopolysaccharide. During extensive screening of vegetable extracts for GSK-3β can be inactivated by phosphorylation of ser- GST-inducing activity in cultured rat liver epithelial ine-9, either through WNT signalling or through acti- RL-34 cells, Morimitsu and colleagues have identified a vation of the PI3K–AKT pathway110. Stabilization of sulphoraphane analogue, 6-methylsulphinylhexyl β-catenin also occurs in the case of either mutation of isothiocyanate (6-HITC), as a key GST-inducer present APC111 or axin112. In addition, a point mutation at the in Japanese horseradish, wasabi (Wasabia japonica or phosphorylation site of the amino-terminal domain Eutrema wasabi Maxim)98. The compound potently of β-catenin turns it into an oncoprotein103 that is induced both class α Gsta1 and class π Gstp1 isozymes resistant to phosphorylation by GSK-3β. in RL-34 cells by stimulating nuclear translocation of Once β-catenin is stabilized, it translocates into Nrf2 and subsequent activation of Are. Oral adminis- the nucleus and interacts with lymphoid enhancer tration of 6-HITC resulted in the induction of hepatic factor (LEF)/T-cell factor (TCF) transcription factors, phase II detoxification enzymes to a greater extent than resulting in transcriptional activation of various sulphoraphane, whereas this induction was abrogated genes. Many of these gene products are involved in in Nrf2-null mice98. In porcine renal epithelial cells, processes such as cell-cycle regulation, cell adhesion both curcumin and CAPE stimulated expression of and cellular development103,113. Genes that undergo Nrf2 by inactivating the Nrf2–Keap1 complex, which transactivation mediated by the β-catenin–TCF/LEF was associated with a significant increase in activity and complex include those encoding c-MYC, cyclin-D1, expression of Ho-1 (REF. 99; FIG. 4). p38 Mapk, which is gastrin, human matrilysin (MMP7), keratin1, upstream of Nrf2, seems to be involved in curcumin- urokinase plasminogen-activated receptor (uPAR), induced Ho1 gene induction. In another study, cur- CD44 and ITF2 (REFS 114,115). Transcription factors cumin increased nuclear translocation of Nrf2, Are such as c-JUN and FRA1 — two components of AP1 DNA binding activity and GCL expression100. It is — are reported to be regulated by the transcriptional notable that both curcumin and CAPE bear an α, activity of the β-catenin–TCF/LEF complex 103. β-unsaturated ketone moiety, and can therefore act as Recently, a TCF4-binding element (TBE) has been Michael-reaction acceptors that are able to modify cys- identified in the COX2-promoter region, and the teine thiols located in Keap1. Sulphoraphane also β-catenin–TCF/LEF complex has been shown to directly reacts with thiol groups of Keap1 (REF. 91) . upregulate COX2 gene expression in human colorectal HT29-APC cells116. β-Catenin β-Catenin is another important target of chemopre- Phytochemicals that target β-catenin ventive phytochemicals. β-Catenin is a multifunctional Several dietary phytochemicals have been shown to protein that was originally identified as a component downregulate the β-catenin-mediated signalling of the cell–cell adhesion machinery. It binds with the pathway as part of their molecular mechanism of cytosolic tail of E-cadherin and connects actin fila- chemoprevention. Curcumin and CAPE inhibited ments through α-catenin to form the cytoskele- tumorigenesis and decreased β-catenin expression in ton101,102 (FIG. 5). It was identified as a component of the the multiple intestinal neoplasia (Min/+) mouse evolutionarily conserved WNT signalling pathway, and model117. Moreover, curcumin reduced the cellular is involved in developmental processes in many organ- leves of β-catenin through caspase-mediated cleavage isms, as well as in tumorigenesis. β-Catenin can also of the protein118. Downregulation of β-catenin expres- function as a transcription factor, and nuclear translo- sion by resveratrol was observed in a human colon cation of β-catenin has been associated with various cancer cell line119. Expression of a β-catenin– human cancers103. TCF4-binding reporter construct was reduced in The cytoplasmic β-catenin undergoes rapid HEK293 cells by EGCG 120,121. Indole-3-carbinol turnover by a large multiprotein complex that consists altered the pattern of β-catenin mutation in chemi- of glycogen synthase kinase-3β (GSK-3β), adenoma- cally-induced rat colon tumours122, inhibited adhe- tous polyposis coli (APC), axin and conductin104,105. sion, migration and invasion of cultured human GSK-3β — either directly or through activation of breast carcinoma cells, and upregulated E-cadherin APC — phosphorylates β-catenin, leading to ubiquity- and β-catenin123. A similar effect was observed with lation followed by proteasomal degradation of tangeretin from citrus124. COX inhibitors have also β-catenin104–106. Recently, the phosphorylation of the been found to suppress β-catenin signalling and serine-45 residue of β-catenin by casein kinase Iε β-catenin–TCF/LEF transcriptional activity 125–127.776 | OCTOBER 2003 | VOLUME 3
  10. 10. REVIEWS Growth factors As upregulation of COX2 promotes tumorigenesis, and β-catenin is found to regulate COX2 expression, WNT ligand modulation of β-catenin signalling could be another RTKs molecular target for chemoprevention by dietary Frizzled receptor E-cadherin phytochemicals. Future directions PTEN Chemoprevention by edible phytochemicals is now PI3K considered to be an inexpensive, readily applicable, β-cat acceptable and accessible approach to cancer control AKT/PKB S P and management. With healthcare costs being a key β-cat Dishevelled β-cat T P issue today, it would be cost-effective to promote the awareness and consumption of phytochemicals as a Ubiquitylation Curcumin cancer-preventive strategy for the general public. CAPE Several nutrients and non-nutritive phytochemi- Resveratrol cals are being evaluated in intervention trials for their COX inhibitors potential as cancer chemopreventive agents. Despite β-cat β-cat significant advances in our understanding of multi- Ub stage carcinogenesis, little is known about the mecha- Indole-3-carbimol APC GSK-3β S P nism of action of most chemopreventive agents. The β-cat Axin/conductin T P chemopreventive effects that most dietary phyto- Ub chemicals exert are likely to be the sum of several dis- tinct mechanisms. Disruption or deregulation of 26S intracellular-signalling cascades often leads to malig- CBP/p300 nant transformation of cells, and it is therefore β-cat important to identify the molecules in the signalling ? network that can be affected by individual chemopre- Cell growth- regulatory genes TCF/LEF ventive phytochemicals to allow for better assessment of their underlying mechanisms. In many cases, the chemopreventive effects of Activation Repression dietary chemopreventives in cultured cells or tissues are only achievable at supraphysiological concentra- tions — such concentrations might not be attained when the phytochemicals are administered as part of Proteasome diet. Furthermore, phenolic phytochemicals are often present as glycosides or are converted to other conju-Figure 5 | Effect of phytochemicals on β-catenin signalling. β-Catenin (β-cat) mediates gated forms after absorption, which might furtherboth growth-factor- and WNT-mediated signalling pathways. The interaction of a WNT- lower the bioavailablity. Both pharmacokinetic prop-ligand with its transmembrane receptor — ‘frizzled receptor’ — recruits dishevelled protein,which inactivates glycogen synthase kinase-3β (GSK-3β) by phosphorylation at serine-9. On erties and bioavailability are key problems in investi-the other hand, interaction of a growth factor with receptor tyrosine kinase (RTK) leads to gating the dietary prevention of cancer and should bethe activation of phosphatidylinositol 3-kinase (PI3K), which, in turn, phosphorylates assessed carefully before undertaking interventionAKT/protein kinase B (PKB). Phosphorylated AKT also inactivates GSK-3β by serine-9 trials with dietary supplements.phosphorylation. A tumour-suppressor protein phosphatase and tensin homologue deleted The development and use of chemopreventiveon chromosome 10 (PTEN) blocks AKT-mediated inactivation of GSK-3β. GSK-3β — a agents for intervention trials involve many scientificcomponent of a multiprotein complex that consists of GSK-3β, adenomatous polyposis coli(APC), axin and conductin — regulates the intracellular fate of β-catenin, which, in its disciplines. With the advances in techniques to assessmembrane-bound form, acts as a component of the cell–cell adhesion machinery and, in its single nucleotide polymorphisms (SNPs), we are nowfree cytosolic form, acts as a signalling molecule. In the absence of a growth factor or WNT more aware of the specific genes that can directly andsignal, GSK-3β phosphorylates cytosolic β-catenin at amino-terminal serine (S) and indirectly contribute to individual differences in thethreonine (T) residues, which is then targeted for ubiquitylation (Ub) by ubiquitin ligase susceptibility to carcinogenesis. When high-riskfollowed by proteasomal degradation. In response to the above stimuli, the inactivation of groups are identified, practitioners might be able toGSK-3β results in cytosolic stabilization of β-catenin. Besides inactivation of GSK-3β,mutation of either APC or axin as well as β-catenin causes its stabilization in the cytoplasm. recommend specific dietary supplements that canStabilized cytosolic β-catenin translocates to the nucleus and binds to T-cell factor modulate or restore the cellular-signalling events that(TCF)/lymphoid enhancing factor (LEF). The β-catenin–TCF/LEF complex acts as a are likely to be disrupted in these individuals. The termtranscription factor and activates transcription of genes that are involved in the regulation of ‘nutragenomics’ has been coined, and much attentioncellular growth processes. Some chemopreventive phytochemicals have recently been is being focused on this relatively new area of research.reported to target β-catenin-mediated signalling pathways. Curcumin downregulates Tailored supplementation with designer foods thatβ-catenin through caspase-mediated degradation of the protein, resulting in decreased consist of chemopreventive phytochemicals — eachDNA-promoter-binding activity of the β-catenin–TCF/LEF complex and reduced levels ofc-MYC protein. Caffeic acid phenethyl ester (CAPE) and resveratrol also attenuate having their own distinct anticancer mechanisms —expression of β-catenin. Epigallocatechin gallate (EGCG) inhibits β-catenin–TCF4 reporter will be available in the near future. These should beactivity and reduces β-catenin protein levels. Indole-3-carbinol shifts the pattern of developed in line with advances in the genetic andβ-catenin mutations, thereby hampering its nuclear translocation. molecular epidemiology of carcinogenesis.NATURE REVIEWS | C ANCER VOLUME 3 | OCTOBER 2003 | 7 7 7