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Introduction
Dietary habits are recognized to be an important modifiable environmental factor influencing cancer risk and tumor
behavior. Although some researchers have estimated that about 30%_40% of all cancer cases relate to dietary habits, the
actual percentage is highly dependent on the foods consumed and the specific type of
cancer[1]. Both essential and non-essential allelochemicals arising from plants, along with zoochemicals occurring in animal products, fungochemicals from
mushrooms, and bacterochemicals from bacteria may be physiologically relevant modifiers of cancer risk. Compounds
encompassing such diverse categories as minerals, amino acids, carbohydrates, fatty acids, carotenoids, dithiolthiones,
flavonoids, terpenes, glucosinolates, isothio-cyanates and allyl sulfurs may influence multiple pathways involved with
cancer, many of which may act additively or synergistically when combined in the human diet.
While optimizing the intake of specific foods and/or their
bioactive components seems a prudent, non-invasive and
cost-effective strategy for reducing the cancer burden, this is far from a simple
process[2]. The magnitude of the problem in
identifying critical dietary components is evident by the literally thousands of compounds consumed each
day[2,3]. Furthermore, the dearth of quantitative information about some food constituents limits the ability to unravel which constituents are most
important. Although it is estimated that humans consume
>5000 individual flavonoids, only a few have been examined
for their cancer protective effects[2]. Unfortunately many
food phytochemicals remain largely uncharacterized and this
can lead to confusion about the true role of diet in
determining health. Interactions between the different components
within a food may explain why isolated components do not
always result in similar biological outcomes to the intact
food[4]. Likewise, interactions among foods and their
constituents may contribute to the overall relationship between
eating behaviors and cancer[4].
Scientifically sound intervention studies must be viewed
as the cornerstone for establishing nutrition
guidance[5]. Unfortunately, the number of long-term intervention studies
that would be needed to adequately define the needs for
bioactive food components is likely to be impractical in terms
of speed of discovery and cost. Alternative procedures that
use validated and sensitive biological markers are needed
that can serve as predictors of those who will benefit most
and those who might be placed at risk, and the minimum
quantity of the food and/or component required to bring
about the intended response. Undeniably, the identification
and selection of appropriate biomarkers will not be an easy
task, but if successful it will have enormous societal
impor-tance.
A biomarker can potentially be any substance, structure
or process that could be monitored in tissues or fluids and
that predicts or influences health, or assesses the incidence
or biological behavior of a disease[5]. Identification of
biomarkers that are on the causal pathway, have a high
probability of reflecting health or the progression to clinical
disease, and have the ability to account for all or most of the
variation in a physiological state or the preponderance of
cases of the specified clinical outcome, have largely remained
elusive[5]. Classes of biomarkers required to adequately
evaluate the physiological significance of functional foods are
probably no different than those used to evaluate
environmental factors that might influence health or disease
risk[5_8]. Thus, biomarkers are needed that reliably evaluate "intake"
or exposure to a specific food or its component, assess one
or more specific biological "effects", and effectively predict
individual "susceptibility" (Figure 1). The interdependence
of these three categories of biomarkers cannot be
over-emphasized.
The selection and validation of intake/exposure, effect
and susceptibility biomarkers must build on sound scientific
investigations and ideally they should be accepted
universally for their predictability. These biomarkers must be readily
accessible, easily and reliably assayed, differentially
expressed in normal and malignant tissues, directly
associated with disease progression, modifiable and _
most importantly _ predictive. The utility of biomarkers capable of
predicting cancer risk will be most valuable if they confer a long
lead-time relative to the onset of the disease.
Molecular biomarkers (the "-omics" approach) will likely
offer the sensitivity and reliability to evaluate dietary
exposures and to provide invaluable insights into behaviors of
specific molecular targets and predictors of individual
responsiveness to dietary change[2]. The study of
nutri-genomics has the potential to identify
definitively which components in foods bring about either positive or negative
consequences, and to clarify their relevant mechanisms of
action and most importantly when they can be manipulated
to reduce cancer risk[9_11]. Knowledge about how diet-induced phenotypic responses depend on an individual's
genetic background (nutrigenetics), the expression of genes
(epigenomics and transcriptomics), changes in the amounts
and activities of proteins (proteomics), and shifts in small
molecular weight compounds (metabolomics) _ collectively
referred to as "-omics" _ will be important in identifying
responders from non-responders[9_11].
Intake or exposure biomarkers
Rapid, accurate and inexpensive methods for assessing
the dietary intake of specific nutrients, both essential and
non-essential, remain a cornerstone, and yet present a
challenge, to determining the relationship between dietary
intake and cancer risk. Errors in estimating food intakes,
interactions among food components and incomplete data
on the content of nutrients limit the usefulness of self-
reported data of food consumption as an "exposure"
biomarker. Because diet is exceedingly complex and may
consist of foods that are consumed intermittently and
irregularly, self-reports of diet may be particularly prone to
measurement error. Food frequency questionnaires (FFQ)
and 24-h recalls are two of the major dietary data collection
instruments that are often used to estimate exposure.
The FFQ is the most commonly used diet assessment
instrument in large observational studies. FFQ are designed
to measure a person's usual dietary intake over a defined
period of time. FFQ are convenient, measure long-term
behavior and are relatively inexpensive. However, FFQ are
limited by knowledge about particular foods and are
hampered by the inability of individuals to accurately report their
food intake retrospectively over a long period of
time[12]. FFQ are prone to underreporting of energy and protein by
20%_50%[13]. Analytical issues compound this problem when
microconstituents are being considered for their
impact on health.
In contrast, 24-h recalls provide in-depth information
about the type and amount of foods consumed; however,
intake on a single day is a poor estimator of long-term usual
intake[12]. Over 20 years ago, Guthrie and
Crocetti[14] found considerable variability in daily intakes of essential
nutrients among individuals over a 3-d period. In their study,
less than 50% of individuals had daily intakes that were within
±25% for most nutrients. Although multiple 24-h recalls or
food diaries overcome these limitations, cost can become
prohibitive in terms of expense, time commitment and
personnel involvement[12,15]. Unfortunately older assessment
tools failed to adequately evaluate dietary supplements as a
contributor to the intake of essential and non-essential
nutrients. Although new assessment instruments and novel
statistical analyses are being developed in an attempt to
overcome these limitations their accuracy and precision
remain uncertain. The Food Propensity Questionnaire has
been developed and uses frequency data as a covariate in
supplementing multiple recalls for estimating the usual
intake of food groups[16]. Conceptually, it may be possible
to model the relationship between 24-h recall probabilities
and FFQ frequencies at an aggregate level and then develop
probabilities of consumption for the individual. Unfortunately, a clear relationship does not appear to hold
for all foods. Finally, the degree of precision that is added
by the use of the Food Propensity Questionnaire appears
somewhat nominal and, therefore, its utility remains
uncertain.
The validity of dietary assessment methods can be
estimated by comparing measurements obtained with different
estimates to biochemical markers reflecting dietary intake.
In the European Prospective Investigation into Cancer (EPIC)
study, correlations between 2 validated biomarkers of intake
and 24-h urinary nitrogen and potassium excretion
correlated better with dietary intake assessed with the 7-d food
diary (r=0.57_0.67 and r=0.51_0.55, respectively) compared
to those from the FFQ (r=0.21_0.29 and
r=0.32_0.34,
respectively)[17]. These data indicate that despite the
increased subject burden and cost, the 7-d diary provided a
better estimate of both protein and potassium intakes
than did the FFQ. However, the low
"r values" reflect a lack of sensitivity, even with the 7-d food diaries, and suggest that
better biomarkers for assessing intake/exposure are needed.
It is certainly reasonable to assume that the intakes of
phytochemicals and other minor dietary constituents will be
even more variable when exposure is estimated based on
self-reported data. Analytical problems associated with the
compositional analysis of foods are in no way a trivial issue.
Furthermore, some dietary components depend extensively
on where the animal was raised or where the plant was grown.
For example, the selenium content of food varies depending
on the selenium content of the soil[18]. Plant genotype may
also modify the concentration of phytochemicals.
Glucora-phanin concentrations in broccoli vary more than 25-fold
depending on the broccoli genotype[19].
Evidence is also emerging that phytochemical
absorption is highly dependent on the food
source[20], as well as the method of processing, as observed with the increased
lycopene availability from processed tomatoes, but the
decreased cancer-protective properties of heated
garlic[21,22]. It is essential that additional attention is devoted not only to
the quantification of all dietary components, but also to
enhancing our understanding about how home or
commercial processing can influence their utilization. Absorption,
metabolism, distribution and excretion may all contribute to
the actual exposure of the target tissue to the bioactive
component (Figure 2). Combining intake assessments with
tissue or fluid concentrations may offer special insights into
how genetics and other environmental factors can influence
absorption and metabolism and can, thus, be useful in
qualitatively approximating an individual's exposure.
Further-more, the reliability of an exposure biomarker will surely
depend on a host of factors, including the time of sampling
relative to when the compound was consumed, the pattern
of intake and the pharmacokinetic properties of the
compound being analyzed.
Effect biomarkers
An "effect" biomarker indicates the presence and
magnitude of a biological response to an intake or exposure. In the
case of foods/bioactive components it would provide an
assessment of the impact of one or more active compounds
on a physiological process and indicate a possible
beneficial or adverse health effect.
Multiple cellular processes appear to account for the
response to bioactive food components in foods or dietary
supplements for influencing cancer growth and/or tumor
behavior (Figure 3). These include, but are not limited to,
carcinogen metabolism, DNA repair, cell proliferation,
programmed cell death, inflammation, differentiation and
angiogenesis. As multiple responses may occur
simul-taneously, it is difficult to determine which is most important
in dictating the overall biological response. Further-more,
the ability of several nutrients to influence the same or
multiple biological processes raises issues about possible
synergy _ as well as antagonistic interactions _ that may occur
within and among foods[4].
DNA instability Human cancers exhibit genomic
instability and an increased mutation rate because of underlying
defects in DNA repair genes. DNA mismatch repair (MMR)
genes have been found to be involved in promoting cytotoxicity, apoptosis, p53 phosphorylation and cell cycle
arrest following exposure to exogenous DNA damaging
agents. Loss of MMR function prevents the correction of
replicative errors, leading to instability of the genome, and
can be detected by polymorphisms in microsatellites. A host
of factors can contribute to DNA instability. Endogenous
agents, including methylating species and reactive oxygen
species arising during normal cellular respiration, can lead to
DNA damage. Some nutrients, such as unsaturated fatty
acids and iron, may influence this process by promoting the
formation of the damaging agents, while other components,
such as some flavonoids and folate, may function to
enhance repair mechanisms[23]. For example, aqueous
fractions of Fushimi sweet pepper have been reported to
increase repair against ultraviolet-induced cyclobutane
pyrimidine dimers in human
fibroblasts[24] and kiwifruit consumption increased DNA repair capacity in human
lymphocytes[25]. Other data point to the essentiality of folate in
maintaining normal DNA synthesis and
repair[26,27]. Dietary components that scavenge activated oxygen species, such
as flavonoids, vitamins E and C and isothiocyanates have
been shown to stimulate repair of oxidative DNA damage.
Moreover, it has been shown that dietary supplementation
with cooked carrots increased the repair of 8-oxodG (an
indicator of oxidative DNA damage) in white blood cells,
whereas a similar amount of α-carotene and β-carotene provided as
capsules had no effect[28]. Some dietary components may
also retard repair as has been suggested following alcohol
exposure[29].
Cellular proliferation and death DNA damage can
arrest cell cycle progression to allow for repair and the
prevention of the replication of the defect or to activate
apoptosis (programmed cell death) to eliminate cells with
catastrophic mutations[30]. Cell homeostasis is regulated by
a delicate balance of proliferation, growth arrest,
differentiation and apoptosis. Alterations in DNA repair, cell cycle
progression and apoptosis are all important molecular
targets for dietary components in cancer prevention.
In general, the growth rate of pre-neoplastic or
neoplastic cells outpaces that of normal cells because of
malfunctioning or dysregulation of their cell-growth and cell-death
machineries[31]. Cell cycle progression is a sequential
process that directs dividing mammalian cells through G1, S, G2
and M phases. Transitions between G1-S or G2-M phases
function as checkpoints to halt cell division if necessary.
Because the balance of interactions among cyclins,
cyclin-dependent kinases (CDK) and CDK inhibitors (CDI)
governs the progression of the cell
cycle[32], perturbation of any of the cell-cycle-specific proteins by dietary components
can potentially affect and block the continuous proliferation
of neoplastic cells and may serve as effect biomarkers.
Dietary components that modulate cell proliferation include
phenolic compounds, such as genistein and
epigallocat-echin-3-gallate, which elicit cell-cycle arrest through the
induction of CDI (p21 and p27) and the inhibition of CDK4,
CDK2, cyclin D1 and cyclin E[33]. Isothiocyanates can also
induce p21 expression and inhibit cell proliferation at the
G2-M checkpoint[34]. Allyl sulfur compounds from allium
foods have been reported to block the cell cycle in the
G2/M phase presumably by inhibiting p34 (cdc2) kinase
activity through changes in cyclin complex formation and
hyperphosphorylation[35].
Apoptosis is one of the most potent defenses against
cancer because this process eliminates potentially
deleteri-ous, mutated cells. Many dietary cancer preventive
compounds, including selenium, epigallocatechin-3-gallate,
phenylethyl isothiocyanate, retinoic acid, sulforaphane,
curcumin, apigenin, quercetin and resveratrol, induce
apoptosis[36,37]. Distinct from the apoptotic events in the
normal physiological process, which are mediated mainly by
the interaction between death receptors and their relevant
ligands[38], many bioactive dietary components appear to
induce apoptosis through the mitochondria-mediated
pathway. Dietary compounds generally induce oxidative
stress, which downregulates anti-apoptotic molecules, such
as Bcl-2 or Bcl-x, and upregulates pro-apoptotic molecules,
such as Bax or Bak[39]. The imbalance between anti-apoptotic
and pro-apoptotic proteins elicits the release of cytochrome
c from the mitochondrial membrane, which forms a complex
with caspase-9 with the subsequent activation of caspases
3, 6 and 7[40]. The activated caspases degrade important
intracellular proteins, leading to morphological changes and
the phenotype of the apoptotic
cells[41]. To enhance this mitochondria-mediated apoptosis, dietary components also
activate pro-apoptotic c-Jun N-terminal kinase (JNK) and
inhibit anti-apoptotic NF-κB
signaling[39]. Thus, potential "effect" biomarkers for the cytotoxic effect of dietary
components on cells may be assessed by measuring their
influence on mitochondrial caspases and other apoptosis-related
proteins. Collectively, common cell cycle and apoptotic
protein targets (biomarkers) relevant to the pathogenesis of
cancer may be useful for not only diagnosis, but also in the
development of preventive strategies, including dietary
change.
Inflammation and immunonutrition The immune
system represents a primary defense against invading
patho-gens, non-self components and cancer cells. Inflammation
is a basic process by which the body reacts to infection,
irritations or other injuries and is recognized as a type of
non-specific immune response. The inflammatory processes,
including the release of pro-inflammatory cytokines [eg
tumor necrosis factor-α (TNF-α), interleukin-1 (IL-1), IL-6,
IL-12, and g-interferon] and the formation of reactive oxygen
and nitrogen species, are critical factors driving this process
and can be influenced by several dietary components.
Although pro-inflammatory actions are usually followed
almost immediately by anti-inflammatory responses (eg IL-4,
IL-10 and TGF-b), excessive production of pro-inflammatory
cytokines may lead to chronic inflammation. Evidence exists
that selected dietary components including conjugated
linoleic acid, long-chain omega-3 fatty acids such as those
in fish oil, butyrate, epigallocatechin-3- gallate, curcumin,
resveratrol, genistein, luteolin, quercetin, and vitamins A and
D may influence unique molecular targets associated with
the inflammatory process[42_47].
The immune system protects against infection by
producing specific antibodies in response to antigens.
Vaccine-specific serum antibody production, delayed-type
hypersensitivity response, vaccine-specific or total secretory
IgA in saliva, and the response to attenuated pathogens are
classic markers that can be influenced by dietary habits.
Markers including natural killer cell cytotoxicity, oxidative
burst of phagocytes, lymphocyte proliferation and the
cytokine pattern produced have also surfaced as potential
predictors of immunocompetence. As no single marker
permits firm conclusions about the ability of diet to modulate
the entire immune system, combining markers appears to be
a suitable strategy. It is also clear that excesses of some
nutrients can enhance the immune system, while other food
components can have detrimental
effects[48].
Angiogenesis Angiogenesis, the development of new
blood vessels from endothelial cells, is a crucial process in
tumor pathogenesis because it sustains malignant cells with
nutrients and oxygen[49]. During angiogenesis, endothelial
cells are stimulated by various growth factors, such as
vascular endothelial growth factor (VEGF) and fibroblast growth
factor (FGF), and are attracted to the site where the new
blood supply is needed by inflammatory cytokines and
chemoattractants[50,51]. Chemotactic migration along this
gradient is, however, possible only through the degradation
of extracellular matrix components[52]. This is accomplished
via matrix metalloproteinases (MMP). Preventing the
expansion of new blood vessel networks results in reduced tumor
size and metastasis and is another mechanism whereby
dietary components inhibit tumor growth. Dietary
components that inhibit angiogenesis include polyunsaturated fatty
acids[53], flaxseed[54] and polyphenols such as
epigallocate-chin-3-gallate, resveratrol, curcumin and
genistein[55_57]. Recently, sphingosine 1-phosphate (S1P), a bioactive
sphingolipid metabolite abundantly stored in platelets and
released upon activation, has emerged as a potent, specific
and selective endothelial cell chemoattractant. Red grape
skin polyphenolic extract prevents and inhibits
angiogenesis in the Matrigel model, decreases the basal motility of
endothelial and cancer cells, and reverses the chemotactic
effect of S1P and VEGF on bovine aortic endothelial cells, as
well as the chemotactic effect of conditioned medium on
human HT-1080 fibrosarcoma, human U-87 glioblastoma and
human DAOY medulloblastoma cells[58]. Inhibition of
S1P-induced and VEGF-induced endothelial cell chemotaxis by
red grape skin polyphenols correlates with a decrease in
early platelet-activating factor
synthesis[58].
Summary of molecular targets Collectively,
overwhelming evidence demonstrates that a variety of nutrients can
influence a number of key intracellular targets that are
associated with the cancer process. A fundamental action of
several bioactive food components is that they serve as
regulators of gene expression and/or modulate gene products.
Transcriptomic or microarray analysis can provide clues
about the mechanisms that underlie the beneficial or adverse
effects of dietary components. Such analysis can identify
important genes and related events that are altered in the
pre-disease state and may, therefore, serve as molecular
biomarkers and/or assist in identifying and characterizing
the basic molecular pathways influenced by food components. Recently, gene expression changes in human
leukocytes after consumption of high-protein or
high-carbohydrate breakfasts demonstrated the potential of
gene-expression profiling in blood to study the effects of dietary
exposure in human intervention
studies[59].
Typically, increasing intensity and duration of the
exposure to dietary components increases the number of gene
expressions that are modified[60,61]. Thus, dose and duration
of exposure become fundamental considerations in
interpreting findings from microrarray studies. In addition, although
most studies are simple snapshots of genomic expression
changes that can help identify important possible targets,
they must be interpreted cautiously because of inherent
biological variability[62]. Determining which one of the targets is
most important in altering tumor growth will not be a simple
task. Likewise, unraveling the multitude of interactions
among nutrients with these key events makes the challenge
even more daunting. Finally, inter-individual differences
probably reflecting genetic polymorphisms can mask the
response to a nutrient and thereby complicate this
undertaking to an even greater extent. Nevertheless, deciphering the
role of diet is fundamental to optimizing health and
preventing disease. Access to this information should help resolve
the inconsistencies within the literature and provide clues to
strategies that may be developed to assist individuals in
improving their health.
Susceptibility biomarkers
Complex gene_environment (including diet) and
nutrient_nutrient interactions are also risk determinants for most
disease states. Furthermore, an individual's physiological
state, such as the stage of the lifespan, will also influence
their disease risk. Thus, an individual's genes,
environmental exposures and physiological state must all be considered
when determining disease risk (Figure 4). The use of
"susceptibility" biomarkers should assist in developing
predictions about whether an individual, or animal, is more likely to
be more sensitive than most other members of a population.
Many of the differences in sensitivity may reflect variation
in genetic backgrounds, namely genetic polymorphisms.
Gene_nutrient interactions The Human Genome Project
suggests that approximately 30 000 genes exist. Most genes
have base sequence differences as a result of single
nucleotide polymorphisms (SNP), insertions and repeats. Some
of the polymorphisms that occur in a gene coding sequence
are of particular interest because they may cause an amino
acid substitution and hence alter the biological function of a
protein. For example, a SNP in the human gene encoding
methylenetetrahydrolate reductase (MTHFR) results in the
substitution of T for C at codon 677 and, thus, a substitution
for alanine with valine. This substitution leads to reduced
conversion of 5,10-methylenetretrahydrofolate to
5-methyl-enetetrahydrofolate, the form of folate that circulates in the
plasma. Individuals with this polymorphism appear to have
increased dietary folate and riboflavin
requirements[63].
Several epidemiological studies have looked at SNP
frequency and folate status in relation to colorectal cancer risk
and most, but not all, suggest that the TT genotype is
associated with decreased risk, particularly in combination with
high dietary folate[reviewed in 64].
Genomic data for human and mouse (including SNP,
expressed sequence tags, gene expression patterns and
cluster assemblies) and cytogenetic information are increasingly
available through a number of databases; these databases
provide opportunities to evaluate genomics as a factor in
explaining variation in response to food components in terms
of human growth, development, performance and health.
Scanning the entire human genome for genetic variations
should provide clues about human evolution. More than
700 genetic variants have already been identified and may
have been targets of natural positive selection over the past
10 000 years and may reflect adaptations to various
conditions, including the availability of a variety of foods
and/or stress conditions. Examples of these websites
include: www.ncgr.org;
www.jgi.doe.gov; www.gmod.org;
www.genome.gov; www.ebi.ac.uk; and
www.nugo.org.
Increasingly, genetic polymorphisms are thought to have
a role in determining the response to foods and their
com-ponents. Unfortunately, while this area is receiving increased
attention, it remains unclear whether these polymorphisms
are directly linked to functional outcomes and disease risk.
Nevertheless, it is certainly plausible that polymorphic
differences have contributed to the inconsistencies among
studies of the health effects of dietary components. In a random
sample of participants in the Alpha-Tocopherol,
Beta-Carotene Cancer Prevention Study (ATBC Study), for example,
the low prevalence of polymorphisms in genes coding for
activation (phase I) enzymes CYP1A1 (0.07) and CYP2E1
(0.02) and the high prevalence in genes coding for
detoxification (phase II) enzymes GSTM1 (0.40) and NQO1
(0.20)[65] may have influenced the outcomes and conclusions of the
study. Furthermore, in a nested case-control study within
the ATBC Study, glutathione peroxidase 1 (hGPX1), a
selenium-dependent enzyme involved in the detoxification of
hydrogen peroxide, was found to have a polymorphism
exhibiting a proline to leucine replacement at codon 198. This
polymorphism conferred a relative risk for lung cancer of 1.8
for heterozygotes and 2.3 for homozygous variants compared
with homozygote wild types[66].
In addition to glutathione peroxidase, genetic
polymorphisms in other anti-oxidant enzymes can influence cancer
risk and the benefits of dietary factors for cancer prevention.
Manganese superoxide dismutase (MnSOD) is the main
antioxidant enzyme in mitochondria. An SNP at codon 16 in
the mitochondrial targeting sequence encodes for an alanine
rather than a valine in the protein. This amino acid change
affects the secondary structure of the protein and the
alanine-containing protein is more efficiently transported into
the mitochondria[67]. Risk of both breast and prostate cancer
appears to be increased by this
polymorphism[68,69]. More-over, in the Physicians' Health Study, high pre-diagnostic
concentrations of several plasma antioxidants (ie carotenoids,
vitamin E, retinol and selenium) were associated with a
significant reduction in aggressive prostate cancer risk only in
individuals containing the alanine-containing
protein[70].
Catalase is another important anti-oxidant enzyme. A
polymorphism in the promoter region leads to the
substitution of T for A at codon 262. This polymorphism is
associated with a dose-response reduction in catalase activity (115.4,
82.1 and 73.5 units/mg hemoglobin) in the CC, CT and TT
genotypes, respectively[71]. The high activity catalase
genotype (CC) was also associated with a 17% reduction in the
risk of breast cancer compared with having at least one
variant T allele[71]. Moreover, differences in catalase activity
by genotype were most pronounced among those in the
highest tertiles of consumption of fruits and
vegetables[71]. These types of studies suggest that some, but possibly not
all, individuals because of genetic polymorphisms in
anti-oxidant enzymes may be at increased risk for cancer and
may benefit most from increased fruit and vegetable
con-sumption.
Thus far, nutrition research has focused mainly on a few
SNP at a time and on the opportunity to modify the
consequences of a SNP, either positively or negatively, by
nutritional intervention. Developments in genotyping
technology make it possible to ascertain hundreds of thousands of
variants in a single sample of DNA. For example, microarrays
containing around 500 000 SNP (500 K arrays) are already
available. These chips are being used in association studies,
such as the Cancer Genetic Markers of Susceptibility
(CGEMS) Project, to identify candidate loci for cancer and
have identified common variants on human chromosome 8q24
that are associated with prostate cancer
risk[72].
Literally millions of SNP occur within the human genome
making it unlikely that a single base change will be found
that is sufficient to account for a number of chronic diseases.
However, because genetic variants are often inherited
together in segments of DNA called haplotypes, which are
shared by a majority of the human population, they may be
useful in deciphering the genetic differences that make some
people more susceptible to disease than others, and likewise
how diet will impact their susceptibility. The International
HapMap Project (http://www.hapmap.org/) may be
particularly useful in teasing out genetic differences that determine
the response to specific foods and their components.
Epigenetic modification can modify gene expression and
disease susceptibility. Evidence already exists that
transcriptional silencing of genes by DNA methylation plays a
crucial role in a number of disease
states[73,74]. Genes involving cell cycle regulation, DNA repair, angiogenesis and
apoptosis are all inactivated by the hypermethylation of their
respective 5'CpG islands. Key regulatory genes _ including
E-cadherin, pi-class glutathione S-transferase, the tumor
suppressors cyclin-dependent kinases (CDKN2) and
phosphatase gene (PTEN), and insulin-like growth factor (IGF-II)
targeted histone acetylation and deacetylation _ are
influenced by DNA hypermethylation. Although folate intake is
recognized to influence DNA methylation patterns, other
nutrients, such as selenium, can also have an
impact[75]. Restoring proper methylation may represent a fundamental
process by which selected nutrients are able to influence
gene expression. Epigallocatechin-3-gallate, the major
polyphenol from geen tea, can inhibit DNA methyltransferase
activity and reactivate methylation-silenced genes in cancer
cells[76]. Recently, Fang et
al[77] have provided evidence that indicates that feeding genistein and related soy isoflavones
can also reactivate methylation-silenced genes, partially
through the direct inhibition of DNA methyltransferase.
These findings need confirmation and the impact of other
dietary constituents in re-establishing normal methylation
patterns deserves additional attention.
Alterations in mRNA, protein and metabolite expression
may also be considered to be susceptibility factors. For
example, if inhibition of a specific molecular target for a
bioactive dietary component is the desired biological
response and an individual has increased expression of the
molecular target, than they may need more of the dietary
component to bring about the desired biological effect. Thus,
transcriptomics, proteomics and metabolomics may provide
information about individual susceptibility.
Nutrient_nutrient interactions Interactions among
dietary components may affect "susceptibility" by modifying
the dose of nutrients that are required to bring about a
physiological effect. This is exemplified by the inability of low
doses of 9-cis-retinoic acid and vitamin D3 to prevent
mammary cancer when given alone, but when given in
combination they were effective[78]. Furthermore, the lower dosages
bypassed potential adverse effects of higher
intakes[78]. The mechanism responsible for this interaction is not known, but
may involve binding of RAR, RXR and VDR, which could
regulate the genes involved with cell proliferation,
differentiation and/or apoptosis. Similarly, low doses of
S-allyl-cysteine and lycopene in combination were able to
suppress the development of MNNG-induced gastric cancer via
modulation of apoptosis-associated proteins (decreased
Bcl-2/Bax ratio and upregulation of Bim and caspases 8 and 3) at
much lower intakes than when given in
isolation[79]. Finally, the combination of vitamin D3 and genistein caused growth
inhibition of DU145 prostate cancer cells at lower, and
biologically achievable, concentrations compared with either
compound alone[75]. Genistein appears to potentiate the
action of vitamin D3 by directly inhibiting CYP24 enzyme
activity and, therefore, increasing the half-life of vitamin D3,
which results in homologous upregulation of cellular VDR
levels[80]. This dual action of genistein leads to enhanced
vitamin D-mediated responses and target gene activation
renders the cells more sensitive to the growth inhibitory and
pro-apoptotic signals of vitamin D3[80].
Dietary components that alter multiple molecular targets
within a specific biological process or pathway may exert
additive or synergistic effects. For example, dietary
components that inhibit different phases of the cell cycle cooperate
to inhibit tumor growth. Quercetin and genistein
synergistically inhibit proliferation of ovarian carcinoma cells by
modifying different stages in the cell cycle and different signal
transduction pathways[81]. Quercetin arrests the cell cycle at
the G1 and S phase boundary, whereas genistein affects the
G2 and/or early M phase. Similarly, quercetin interacts
synergistically with resveratrol to cause transient cell cycle
arrest in human leukemia cells[82] and
epigallocatechin-3-gallate and curcumin synergistically inhibit the growth of normal,
premalignant and malignant human oral epithelial
cells[83]. Whereas EGCG blocked cells in G1, curcumin blocks cells in
S/G2M. Thus, synergistic interactions between/among
dietary phytochemicals are likely to contribute to inhibition of
cell proliferation.
A critical unanswered question is what additive or
antagonistic responses occur among dietary components that
have the same molecular target. For example, organosulfur
compounds in garlic and sulforaphane in broccoli can both
induce expression of detoxifying enzymes via the binding of
the transcription factor Nrf2 to the anti-oxidant response
element (ARE). It remains unclear if broccoli would illicit
protective effects if Nrf2 was maximized by consumption of
organosulfur compounds. Additional information is needed
to determine the biological significance of nutrient_nutrient
interactions and their influence on susceptibility biomarkers.
Physiological state Cancer susceptibility can also be
modified by the timing and duration of exposure to dietary
components. Despite its well-recognized limitations, birth
weight has been used widely as a summary measure of the
normality of intrauterine growth[84]. There is evidence that
higher birth weight may increase the overall risk of cancer in
adulthood in both men and women[85]. Thus, individuals
who are born overweight may have increased susceptibility
regardless of genetics or future environmental exposures.
The timing of dietary exposure may also influence
susceptibility. In a rat model of carcinogen-induced
mammary cancer, limiting exposure to dietary genistein, the
primary isoflavone of soy, to the prenatal or adult periods does
not predispose or protect against mammary
cancer[86]. In contrast, exposure to dietary genistein during the
prepubertal and prepubertal plus adult periods protected against
mammary cancer[86]. Similarly, a case-control study in Shanghai
has shown an inverse relationship between adolescent
13_15-year olds, soy food intake and breast cancer incidence
later in life[87]. These data suggest that the consumption of
soy during the prepubertal period, compared to other stages
of the life cycle, may be more efficacious against mammary
cancer development.
Another important consideration is whether the effects
of dietary components persist for extended periods. Results
from the General Population Trial in Linxian, China,
demonstrated that individuals who received a supplement
containing beta-carotene, vitamin E and selenium, had a 13%
reduction in cancer mortality. Post-intervention follow up in
dicated that the beneficial effects of the supplement were
still evident up to 10 years after termination of the
supplementation[88]. Moreover, these benefits were consistently
greater in participants who were younger (<55 years) at the
beginning of the intervention and cancer risk appeared to
increase in those who started supplement use after the age
of 55 years compared with those who started supplement
use when they were <55 years[88]. Evidence for a carry-over
effect of nutrients also comes from observations made in a
cohort study in Washington County, Maryland, USA. In
this study an inverse relationship between circulating
25(OH)D, a biomarker of vitamin D exposure, and colon cancer
was observed in the first 8 years after the blood sample
collection, but no association was observed in cases
diagnosed 10_17 years after the sample
collection[89]. Results such as these suggest that an individual may still have
decreased cancer susceptibility after discontinuing exposure
to beneficial dietary components, but the positive effects
are unlikely to last indefinitely. Regardless, a better
understanding of the temporal relationship between exposure to a
dietary component, whether as foods or supplements,
warrants additional attention.
For most cancers, increased age is a significant
susceptibility factor[90]. The cancer-prone phenotype of older
humans may reflect the combined effects of accumulation of
DNA damage, increased epigenetic gene silencing, telomere
dysfunction and altered stromal
milieu[90]. Because most cancers occur during the later stages of life, a better
understanding of the age dependency of diet_genetic interactions
is needed.
The response to dietary components may also depend
on the presence of preneoplastic lesions or tumors at the
time of intervention. Animal studies using chemical and
genetically predisposed models provide strong evidence for
a causal relationship between folate depletion and increased
cancer incidence, as well as a dose-dependent protective
effect of folate supplementation[reviews in
91]. However, animal studies have also shown that the dose and timing of folate
intervention are critical in providing safe and effective
cancer prevention[91]. Exceptionally high supplemental folate
levels and folate intervention after microscopic neoplastic
foci are established in the colorectal mucosa can promote
rather than suppress colorectal
carcinogensis[91]. Such
evidence suggests that the optimal dose of folate for cancer
prevention may vary among individuals and may depend on
whether or not microscopic lesions are present.
Summary
Dietary modifications and interventions have the
potential to significantly lower cancer risk and its associated
complications. Validated biomarkers would be invaluable
tools to facilitate research in this area and to identify those
who would respond to dietary change. Biomarkers of
exposure would allow evaluation as to whether sufficient intakes
are achieved to bring about a biological response in a
particular cellular process. Biomarkers of biological effect will
provide insights into sites of action and, thus, mechanisms
of action of dietary components. In addition, biomarkers are
needed to identify individuals susceptible to specific dietary
exposures. Despite promising recent research, there remains
an enormous need for defining the applicability of
non-invasive biomarkers in humans. The application of molecular
genetics is likely to assist in achieving greater precision in
identifying responsive individuals. It is very likely that a
suite of biomarkers, rather than a single measure, will be
needed to adequately evaluate the impact of altering dietary
intakes on overall health and disease prevention.
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