Emerging Science
Hormesis and synergy: pathways and mechanisms of quercetin
in cancer prevention and management
Ashley J Vargas and Randy Burdnure_301 418..428
Quercetin is a unique dietary polyphenol because it can exert biphasic
dose-responses on cells depending on its concentration. Cancer preventative effects
of quercetin are observed at concentrations of approximately 1 40 mM and are likely
mediated by quercetin's antioxidant properties. Pro-oxidant effects are present at
cellular concentrations of 40 100 mM. However, at higher concentrations, many
novel pathways in addition to ROS contribute to its effects. The potent bioactivity of
quercetin has led to vigorous study of this compound and revealed numerous
pathways that could interact synergistically to prevent or treat cancer. The effect of
intake and concentration on emerging pathways and how they may interact are
discussed in this review.
© 2010 International Life Sciences Institute
INTRODUCTION effects, which again rely on the concentration of quercetin
at the tissue site. Quercetin s hormetic nature makes it
Quercetin is a dietary polyphenol that is readily found in
well-suited to use in cancer prevention efforts; these
a variety of foods and is consumed daily. The tremendous
efforts would likely involve lower and long-term con-
growth in the study of this bioactive compound has
sumption of quercertin-containing foods, and/or supple-
revealed numerous pathways that could possibly interact
mentation or therapeutic administration in combination
to prevent or treat diseases such as cancer, so a review of
with conventional therapies.Although this review focuses
the recent literature utilizing this compound is pertinent.
on quercetin in relation to cancer, it is generally under-
Quercetin also has a unique ability to act as an antioxi- stood that the same pathways could be applied to other
dant or a pro-oxidant depending on its concentration,
disease states as well.
which is indicative of its hormetic properties.1 For the
purpose of this review, hormesis is defined as a biphasic
dose-response whereby low doses of quercetin result in a
CENTRAL ROLE OF REACTIVE OXYGEN SPECIES IN
given effect (antioxidant properties) and higher doses
QUERCETIN ACTIVITY IN CANCER
result in another effect (pro-oxidant properties). Given
this assumption, there are two fundamental factors that The biphasic oxidation properties of quercetin are likely
impact quercetin s bioactivity as either an oxidant or an beneficial in cancer prevention and therapy because dif-
antioxidant. First, and arguably most important, is quer- ferent concentrations of quercetin counter the transfor-
cetin s tissue bioavailability and digestion process. Second mation and growth processes of cancer.1 Malignant
is the concentration and isoform or conjugate form of tumors result from uncontrolled cell growth due to muta-
quercetin in the target tissue. In this review, multiple tions. Mutations are a result of DNA damage, which is
effects of quercetin are proposed that are concentration commonly incurred through exposure to reactive oxygen
dependent; this implies a dependence on the entire meta- species (ROS). Quercetin is able to donate electrons
bolic process of this flavonoid. It is also proposed that to ROS2 and thereby reduce their ability to damage cellu-
multiple pathways could interact to produce synergistic lar DNA.3 This is the primary mechanism by which
Affiliation: AJ Vargas and R Burd are with the Department of Nutritional Sciences at the University of Arizona, Tucson, Arizona, USA.
Correspondence: R Burd, Department of Nutritional Science, University of Arizona, Shantz Building, Room 301, 117 E. 4th Street, Tucson,
AZ 85721, USA. E-mail: [e-mail], Phone: [phone]; Fax: [phone].
Key words: estrogen receptor, HSPs, P53, quercetin, ROS
doi:10.1111/j.1753-4887.2010.00301.x
418 Nutrition Reviews® Vol. 68(7):418 428
quercetin exerts antioxidant and chemopreventive effects other hypotheses, including the idea that some hydrolysis
on the cell.3 Typically, this effect is seen at cellular quer- occurs in the small intestines, via both b-glycosidase and
cetin concentrations in the range of 1 40 mM, which lactase phlorizen hydrolase (LPH).9 Despite differences of
could likely be achieved by diet.4 However, after a tumor opinion, it is generally accepted that bioavailability
has formed, quercetin could continue to have beneficial depends on the location and type of sugar group attached
anti-tumor effects at higher doses by exerting cytotoxic to quercetin.8 Most likely, the digestion and absorption
effects. Quercetin is able to increase oxidative stress and of quercetin occurs through a combination of the pro-
cytotoxicity in tumor cells, usually at concentrations posed pathways, depending on which form the quercetin
greater than 40 mM; it is able to do this by becoming an is in at a given point in time.
ROS itself and by increasing damage or apoptotic path- Hydrolysis of quercetin by b-glycosidase results in
ways in the transformed cell.2,3 These benefits rely on ROS different metabolites of quercetin depending on what
and the quercetin concentration to produce either anti- the original glycoside was (i.e., where the glycosidic bond
or pro-oxidant effects. There are various pathways and was located and what type of sugar was attached). These
mechanisms that can interact and these are described in metabolites include not only free quercetin, but also con-
more detail below. In addition, because biomedical jugates such as glucuronides, O-methylated products, and
research must be translatable to real-life situations, this sulfate forms.8 This conjugation of quercetin is reported
review begins with an assessment of the bioavailability to occur throughout the processes of digestion and
and metabolism of quercetin. Since much of the research absorption.8 In animals, it appears that quercetin and its
discussed in this review has only been conducted in vitro metabolites are transported unevenly throughout the
or in cell culture, an attempt is made here to link these body.10 Animal studies have also shown that blood con-
studies with biologically relevant concentrations in tains mostly quercetin metabolites after quercetin inges-
humans. tion,8,10 whereas only the organs involved in quercetin
metabolism (i.e., kidney, liver, and intestines) can contain
significant amounts of free quercetin in addition to
ABSORPTION AND METABOLISM OF QUERCETIN
methylated forms.10 However, few studies have focused
Quercetin is consumed daily by millions of people on detecting quercetin concentrations at target tissues
through nuts, teas, vegetables, and herbs in the diet.3 It is and further research is greatly needed in this area. The
also available as a commercial dietary supplement, and it findings of one study conducted in pigs indicated that the
is now being included in functional foods. Quercetin is kidney, liver, and jejunum had concentrations of querce-
generally recognized as safe in oral dosages of 1,000 mg/ tin between approximately 2.0 and 6.0 mM/L.10 Human
day or in intravenously administered dosages of 756 mg/ studies are not available to confirm this finding4; however,
day.5 Up to 60% of orally ingested quercetin is absorbed,5 both human and animal studies suggest that quercetin s
and the average dietary intake of quercetin is somewhere distribution and absorption depend on its form.4,10
between 6 and 31 mg daily (not including supplement/ Further, studies have shown that both the bioavailability
intravenous use).6 Quercetin is part of the flavanol family and other intestinal contents can affect absorption of
and it is normally found in the glycosylated form.7 Diges- quercetin and its derivatives.8
tion of most dietary quercetin, in the form of quercetin The reduction-oxidation potential of a quercetin
glycosides, begins in the oral cavity with some cleavage of molecule is also dependent on quercetin s form. For
the glycosides catalyzed by b-glycosidases (Figure 1).7 example, non-catechol containing structures do not
Some of quercetin s aglycoside form is absorbed in the chelate oxidative metals as well as those that do contain
mouth as well.7 catechol.8 Given the immense variability in quercetin
There is some disagreement as to the exact post-oral metabolism, it is tremendously complicated to assess
cavity metabolism of this substrate; however, a few pos- quercetin s direct ability to exert pro-oxidant and antioxi-
sibilities exist (Figure 1). It is likely that the colonic micro- dant affects in the body. Additionally, there are too many
flora hydrolyze the glycoside-form of quercetin to the factors to provide a complete comprehensive review of
more active aglycone quercetin. Once aglycosylated, the the literature involving quercetin metabolism. Thus, this
molecule becomes more lipophillic and can then be review focuses on studies that have examined oral supple-
absorbed into the epithelial cells of the colon.8 Another mentation and/or dietary intake of quercetin versus
possibility is that some of the glycosidic quercetins are blood concentrations or tissue concentrations. Since it is
absorbed directly, particularly those that are bound with conceivable that long-term consumption and chronic
glucose.8 It is also probable that colonic microflora quercetin blood concentrations will eventually infiltrate
ferment quercetin into phenolic compounds and carbon tissues, a general assumption is made that it may be pos-
dioxide.5 Both the carbon dioxide and the phenolic com- sible to achieve levels of quercetin in tissues and tumors
pounds are then expelled from the body.5 There are yet that are somewhat near those measured in blood. Also,
Nutrition Reviews® Vol. 68(7):418 428 419
Figure 1 Schematic of possible pathways by which quercetin is digested, absorbed, metabolized, and excreted in the
human body. Typically, quercetin glycoside is ingested orally and is then probably partially digested in the oral cavity. Surplus
quercetin is then digested and absorbed at multiple sites along the gastrointestinal tract. During absorption, or shortly
thereafter, quercetin undergoes modification and then enters the circulatory system in a conjugate form. The circulatory system
delivers quercetin to other tissues in mostly conjugated forms, and once quercetin reaches the target tissues it can likely be
converted back into the parental compound.
given that limited data are available on quercetin concen- diet. Egert et al.4 supplemented 18 men and 18 women for
tration and form in human organs or tissue and that 2 weeks with various levels of the oral quercetin aglycone.
many of the conjugate forms of quercetin are converted The study participants had no difficulty absorbing
back to the parental compound by cellular processes, only dosages of up to 150 mg/day but the researchers did find
those mechanisms involved in free quercetin action at the variations in individual blood serum concentrations that
target tissue will be examined.11 It should be noted that were independent of fat mass or sex.4 The blood plasma
when discussing the high concentrations of quercetin measurements for average total quercetin levels from
that would be required for therapeutic effect, it is likely 50 mg/day, 100 mg/day, and 150 mg/day supplementa-
that high-dose supplementation or more direct forms of tion were 145 nmol/L, 217 nmol/L, and 380 nmol/L,
administration would be required. While this is an over- respectively, after only 2 weeks of daily ingestion.4 Two
simplification, the diverse nature of the polyphenols well-known, specific quercetin metabolites (isorham-
makes it necessary to focus on the mechanisms of the metin and tamarixetin) were increased to between 9 and
parental compound in order to to further understanding 23 nmol/L after treatment with the various dosages of the
of its conjugates and how all factors combined will ulti- oral quercetin aglycone.4 These findings agree with those
mately impact the outcome of using quercetin for cancer from other human quercetin absorption studies.4,12 It
prevention and treatment in humans. should be noted, however, that the measurement of total
There are very few human studies that have evalu- quercetin includes all detectable conjugates, which can
ated the absorption of quercetin, and most of them were then be converted to free quercetin in the cell.11 Because
performed with low doses that could be achieved through the preventative effects of free quercetin are seen in vitro
420 Nutrition Reviews® Vol. 68(7):418 428
at approximately 1,000 40,000 nmol/L (1 40 mM), for quercetin is able to prevent oxidative DNA damage and
antioxidant effects, it is likely that these concentrations increase DNA repair at lower dosages.
could be achieved through diet or, more likely, dietary Quercetin also has the ability to work synergistically
supplementation of quercetin.4 However, the cancer- with other antioxidant systems in the body in order to
treating pro-oxidant effects are not commonly seen decrease oxidative stress.16 When quercetin exerts its anti-
until cellular concentrations reach 40,000 to above oxidant power, it can advance to the semiquinone or even
100,000 nmol/L (40 >100 mM). There are animal studies the 0-quinone state.16,17 In these highly oxidized states,
that support the possibility of reaching higher concentra- quercetin is potentially damaging to the cell and activates
tions in vivo. Silberberg et al.13 found that the combined another antioxidant pathway involving glutathione
plasma concentration in rats, after oral consumption of (GSH).17 Kim et al.17 recently examined the relationship
45 47 mg/day for 2 weeks, was approximately 60 mM. between oxidized quercetin and GSH in a human
Another possibility is to administer quercetin intrave- hepatoma cell line (HepG2).17 Their findings indicate that
nously.14 A phase I clinical study found that individuals 10 and 100 mM doses of quercetin led to antioxidant
with a cancer diagnosis could tolerate acute serum levels affects, but that exposure to 100 mM quercetin for longer
of 200 400 mM.14 Although more research is needed, it than 30 min led to pro-oxidant/pro-apoptotic effects.17
appears to be physiologically possible to meet the ranges More specifically, the data indicated that quercetin is able
required to both prevent and potentially treat carcino- to chelate reactive metal ions that produce ROS, react
genesis with quercetin. with hydrogen peroxide to reduce ROS, and use GSH-
mediated reduction in order to return ROS to their
reduced states.17 This cooperativity with GSH is likely one
ANTIOXIDANT MECHANISMS OF QUERCETIN
mechanism by which quercetin can protect the cell from
Quercetin is able to react with ROS and chelate ROS- mutagenesis. On the other hand, quercetin may be able to
producing metal ions, both of which allow for decreased cause cellular damage when it is administered in a long-
oxidative DNA damage.8 Preventing this DNA damage is term high dose.
believed to be the general mechanism by which quercetin Animal models are frequently the vehicle for mea-
is able to prevent tumorigenesis.8 In particular, it is suring overall antioxidant status after treatment with
known that quercetin s hydroxyl groups have electron- quercetin. Santos et al.18 fed mice 4.2 mg of quercetin
accepting capacity when they are in the semiquinone state daily for 3 weeks and then measured blood values of
and that its catechol group is the structure that confers the quercetin metabolites against control mice. The primary
ability to chelate metal ions.8 The addition of sugar mol- metabolites found in the blood were glucuronide sulfate
ecules to form quercetin glycosides can obstruct both of conjugates of isorhamnetin at a concentration of
its antioxidant activities. Therefore, the aglycosylated 4.2 mM.18 The chemical structures of these conjugates
form is usually of higher antioxidant potency than the inhibit some of the antioxidant capacity as compared to
glycoside form, depending on where the sugar molecule is the parent compound. Thus, the bioactivity of quercetin
attached.8 In this review, references to quercetin indicate conjugates is lower than that of the parent compound.18
the free, aglyconated form. This decrease in bioactivity was supported by the
A recent study looking at quercetin s antioxidant unchanged antioxidant activity when the experimental
mechanism in colorectal adenocarcinoma cells (Caco2) and control blood samples were compared.18 Similar null
found that treatments consisting of 1 mM concentrations results were found in humans when serum quercetin
of quercetin led to decreased double-stranded DNA metabolite concentrations reached 1.031 mM after
breaks, but that higher concentrations of quercetin onion consumption.19 Like in the murine study,18 the
increased double-stranded DNA breakage.15 Recall that researchers in the human study also attributed their null
double-stranded DNA breakage is a major source of results to the blood concentration probably being lower
mutagenesis and subsequent malignancies in cells.15 than the threshold needed to significantly change anti-
However, increased double-stranded breaks can also lead oxidant biomarkers.19 However, different results were
to increased apoptosis, as described later in this review. obtained when higher dosages, approximately 20 mg
Congruently, this group found decreased hydrogen quercetin, were administered to mice intragastrically.18,20
peroxide-induced single-strand DNA breakage in Caco2 These acutely exposed mice achieved a 13.2 mM serum
cells pretreated with low-dose quercetin as compared to concentration of quercetin metabolites, which was
cells that were untreated.15 Additionally, it was deter- expectantly higher than the concentration in the previ-
mined that both low (1 mM) and high (100 mM) quercetin ously discussed lower-dose studies.20 The higher concen-
treatments led to increased expression of human tration was enough to increase the antioxidant capacity
8-oxyguanine DNA glycosylase (hOGG1). The hOGG1 of the treated mice, at 119 nmol Trolox equivalents/mL
protein is involved in repairing DNA.15 This suggests that plasma, relative to the control, at 48 nmol Trolox
Nutrition Reviews® Vol. 68(7):418 428 421
Figure 2 Map of several pro-apoptotic pathways triggered by quercetin concentrations greater than 40 mM. Quercetin
can generate increased cellular ROS, which then increases tumor suppressor proteins and leads to cell death via the mitochon-
drial pathway. Quercetin can also initiate cell death via the death domain pathways. Lastly, quercetin contributes to the
inhibition of proteins that encourage proliferation. Note: Arrows do not always indicate a direct mechanism of action.
equivalents/mL plasma.20 These studies illustrate how MITOCHONDRIAL APOPTOTIC PATHWAY
dependent quercetin s antioxidant capacity is on both the (P53-DEPENDENT AND -INDEPENDENT)
concentration and form of quercetin in the blood and,
The mitochondrial apoptotic pathway is initiated via Bcl-
presumably, target tissues.
2-associated X protein (Bax) and/or Bcl-2 homologous
antagonist/killer (Bak) proteins that bring about an
increase in the mitochondria outer-membrane pore
PRO-OXIDANT MECHANISMS OF QUERCETIN
size. This allows for cytochrome C, among other pro-
As described above, quercetin is not only an antioxi- apoptotic proteins, to leak out into the cytoplasm. When
dant1; it can also become a pro-oxidant at high concen- cytochrome C is freed into the cytoplasm, it is able to
trations or for longer incubations at the greater combine with apoptotic protease activating-factor 1
concentration. The present review of the literature indi- (APAF-1) and undergo a conformational change, thus
cates that, in general, quercetin is able to act as a pro- forming the apoptosome. The apoptosome then enlists
oxidant at concentrations greater than 40 mM, which is caspase-9 in order to activate the so-called executioner
in agreement with Watjen et al.1 Although cytotoxicity proteins, caspase-7 and caspase-3. Cell death is subse-
may not be a desirable outcome in healthy cells, it would quently carried out by these caspase proteins (Figure 2).23
be greatly beneficial in tumor cells. Thus, if quercetin Quercetin is a known inducer of apoptosis in multiple
was supplemented at high does or administered intrave- cancer cell lines when administered in doses of 40 50 mM
nously, like other chemotherapeutic drugs, it may be or greater concentrations.24 26 Larger doses of quercetin
possible to use this pro-oxidative tendency in order to and longer exposure times lead to decreased cancer cell
initiate apoptosis in humans with cancer. Therefore, viability. It has been proposed that the mitochondrial-
quercetin could likely be used as an adjuvant to current mediated cell-death pathway is a mechanism used by
chemotherapies, and if quercetin is activated (oxidized) quercetin in order to induce apoptosis.25
by enzymes in tumor cells, the dose needed for the pro- Examples of quercetin s antiproliferative effect are
oxidant or anti-tumor responses could be considerably largely documented as being mediated through the
lower.21,22 Recently discovered mechanisms by which induction of P53.24 26 This tumor-suppressor protein can
quercetin is able to bring about advantageous cell death activate Bax and initiate cell death.27 Recently, Tan et al.24
are discussed below. investigated protein expression and cell status of a human
422 Nutrition Reviews® Vol. 68(7):418 428
hepatocellular carcinoma cell line after treatment with lines. The death-domain pathway involves activation of
40 120 mM dosages of quercetin. Tan et al.24 found that FAS receptor, then FAS-associated death domain, and
quercetin induced increases in P53, while decreasing the subsequently caspase-8.30 Caspase-8 then induces
antiapoptotic Survivin and Bcl-2 proteins.24 Survivin acts caspase-3, which triggers cell death.30 Quercetin can work
at the caspase level to prevent apoptosis while the ability alone or in conjunction with other molecules in order to
of Bcl-2 to prevent mitochondria-directed apoptosis is bring about cell death (Figure 2).31 Quercetin even has
dependent on the relative amounts of Bax present. some ability to differentiate between normal versus
Further, the researchers found amplified caspase-9 and its malignant cells.31 Below are some examples of novel
downstream proapoptotic substrate, caspase-3, activity.24 mechanisms by which quercetin can bring about tumor
Given that Bcl-2 is a negative regulator of apoptosis, and cell death.
that caspase-3 is a positive regulator of apoptosis,24 one In addition to elucidating quercetin s involvement in
can conclude that the P53-dependent induction of the the mitochondrial pathway, Chein et al.28 found evidence
mitochondria-mediated pathway is what allows quercetin that quercetin may also induce a separate, p53-
to induce cell death inthis cell line.24,26 independent apoptotic pathway known as the death-
In a similar study performed in human breast cancer receptor or death-domain pathway. In MDA-MD-231
cells (MDA-MB-231), P53 and the mitochondria- cells, Chein et al.28 found an increase in FAS and
mediated cell-death mechanism were also implicated as caspase-8 activation after treatment with 250 mM of quer-
the mechanism by which quercetin is able to induce cetin. This indicates that high doses of quercetin likely
apoptosis. Chein et al.28 observed an increase in P53, provoke cell death through the cell-death-receptor path-
caspase-9 activation, caspase-3, cytochrome c, and apop- ways, in addition to the mitochondria-dependent cell-
tosis in MDA-MB-231 cells treated with 200 250 mM death pathway in the same cell line.
quercetin in vitro. In addition, this group measured and Quercetin may also work synergistically (Figure 2)
found a decrease in mitochondrial membrane potential with other death-domain stimulators, like tumor necrosis
after treatment with quercetin.28 Adecreaseinmembrane factor a (TNF-a)-related apoptosis-inducing ligand
potential would be consistent with the  leaky mitochon- (TRAIL), to bring about cancerous cell death.31 Siegelin
drion evident in mitochondria-mediated apoptosis.28 et al.31 recently found that the coadministration of 100 or
Even in the absence of P53, quercetin is able to exert 200 mM of quercetin will lead to the sensitization of
its mitochondria-mediated cell death via the presence of glioma cells (U87-MG, A172, U251, U373, and LN229) to
P63 and P73.26 Both P63 and P73 are similar enough in TRAIL and, consequently, apoptosis. Glioma cells are
structure to P53 that they are able to increase transcrip- notoriously resistant to TRAIL-induced apoptosis, and
tion of Bax.26 Recently, Zhang et al.26 demonstrated the this group found that neither quercetin nor TRAIL alone
dose-dependent cytotoxicity of quercetin to human caused significant cell death in their cell lines at doses
esophageal squamous cell carcinoma cell line (KYSE-510) below 300 mM.31 Combinations of quercetin and TRAIL,
that is p53 mutated in vitro. The direct mechanism for however, produced apoptosis, as measured by the pres-
apoptosis was provided by the observed increased cleav- ence of cleaved poly(ADP-ribose) polymerase and flow
age of procaspase-9 and caspase-3 after KYSE-510 cells cytometry in all examined gliomas, except U373. This is
were treated with 80 mM quercetin.26 Quercetin is there- significant given that TRAIL selectively kills only cancer-
fore able to initiate apoptosis via the mitochondrial ous cells while leaving healthy tissue alive.31,32 To confirm
pathway involving activation of caspase-3 downstream the death-domain-pathway activation, Sieglin et al.31
from caspase-9, as long as a functioning p53-like protein measured and found an increased active caspase-8 cleav-
is activatable.26 Of interest was another finding in this age product in U87-MG, A172, and U251 but not in the
same experiment involving the quercetin-provoked other cell lines. This indicates the death domain pathway
increase in expression of p53-inducible gene 3 (PIG3).26 is an active player in the apoptosis seen in these cells.
PIG3 is quinone oxidoreductase and is responsible for the Additionally, this study measured caspase-level inhibitors
NADP-dependent reduction of quinones, like quercetin. of apoptosis. X-link inhibitor of apoptosis proteins
It is thought that PIG3 induces cell death by enzymatically (XIAP) is similar to survivin in that it is antiapoptotic and
upregulating ROS, but the exact mechanism has not been that both are upregulated in resistant glioma cells.31,32
elucidated fully.29 XIAP, along with survivin, were decreased after the cells
were exposed to both quercetin and TRAIL, again except
in the U373 cells.31
SYNERGISTIC EFFECTS OF QUERCETIN IN APOPTOSIS
As stated previously, both mitochondria- and death-
Higher dosages of quercetin can trigger apoptotic cas- domain-directed apoptosis has been noted to occur in
cades by multiple mechanisms and via both the mito- the same cell line simultaneously. In addition to the
chondrial and death-domain pathways in various cell classic death-receptor-mediated pathway induced by
Nutrition Reviews® Vol. 68(7):418 428 423
TRAIL, Sieglin et al.31 found evidence implicating the QUERCETIN AND PROTEIN CHAPERONE INHIBITION
mitochondrial pathway as well. The Bcl-2-interacting
domain (Bid) is involved in the mitochondrial apoptotic Quercetin promotes apoptosis by interfering with prolif-
pathway and leads to the inhibition of the downstream eration and cell maintenance pathways as discussed above
caspase-inhibiting survivin and XIAP proteins.32 (i.e., Bcl-2, survivin, and XIAP inhibition). Specifically,
Upregulation of Bid was seen in U87-MG and A172, however, emerging science is indicating that quercetin-
indicating that, at least in those cell lines, both types of directed protein chaperone inhibition may play a large
apoptosis are involved in quercetin/TRAIL-induced cell role in the stimulation of cell death.38,39 Protein chaper-
death.31 ones are responsible for the correct folding and mainte-
In addition to TRAIL, quercetin has been shown to nance of proteins in the body. When protein chaperones
work with the estrogen receptor a (ER-a) in order to are unable to perform their duties, cell functionality is
induce cytotoxicity in some cervical cancer cell lines.33 decreased and cell death is plausible.39 Heat shock protein
In a recent study by Galluzzo et al.,33 doses as low as (HSP) chaperones, specifically, are unregulated in some
1 mM, and up to 100 mM, were shown to decrease the tumor cells and initiated by ionizing radiation.39 Querce-
number of cells in the human cervix epitheloid carci- tin is able to inactivate these protein chaperones, seem-
noma cell line (HeLa) that were ER-a-positive but not ingly by its ability to inhibit the kinases that aid in HSP
the number of HeLa cells that were ER-a-delete. Due to induction (Figure 3).39 This ability is currently being
evidence from research on a related bioflavonoid, nar- explored as an anti-cancer mechanism and is discussed
ingen, Galluzo et al.33 hypothesized the apoptosis was a below.
result of quercetin stimulating the ER-a-P38/mitogen- Expression of HSP70 is stimulated by radiation-
activated protein kinase apoptotic pathway. Although induced heat in tumor cells. The heat induces the phos-
this pathway only seems to profoundly affect certain cell phorylation of heat shock transcription factor 1 (HSF1)
lines, it leads to increased FAS (pro-death domain by either of two kinases: casein kinase 2 (CK2) and
pathway) and increased Bax (pro-mitochondrial-cell- calcium/calmodulin kinase II (CamKII). Once phospho-
death pathway).34 ER-a can initiate both estrogen- rylated, these kinases activate HSF1, which catalyzes the
triggered proliferative and pro-apoptotic pathways.33 transcription of HSP70. A novel experiment by Wang
Since quercetin is considered a phytoestrogen, it is likely et al.39 in Jurkat cells (immortalized T-lymphocytic cells)
that this estrogen-mimicking ability is able to prevent demonstrated that quercetin is able to inhibit the kinase
the ER-a from initiating prosurvival pathways by com- activity of CK2 and CamKII, subsequently decreasing
peting with estrogen for binding at the receptor. HSP70 expression and increasing tumor sensitivity to
However, the non-estrogen-dependent phosphorylation radiation.39 This effect, however, was not seen in human
of p38 it still able to proceed and initiate the proapop- HeLa cells.39 Confounding the potential of quercetin s
totic pathways.33 ability to inhibit HSPs is also the finding in this same
Despite quercetin s ability to interact with ER-a, it experiment that quercetin somehow contributes to the
preferentially favors binding to estrogen receptor b phosphorylation and undesired activation of HSP27.39
(ER-b) over ER-a.35 Sotoca et al.35 recently explored the This finding is consistent with other findings that quer-
impact that quercetin s receptor affinity has on apoptosis cetin s actions are cell-type dependent, and Wang et al.
in breast cancer (T47D-ER-a) and osteosarcoma (U2OS- were resolute enough to elucidate two quercetin deriva-
ER-a and -ER-b) cell lines. Upon administration of tives that both inhibited HSP70 expression while not acti-
ascorbate-stabilized quercetin, this group saw increased vating HSP27.39
quercetin-ER-b binding and apoptosis at concentrations Quercetin has also been shown to decrease HSP90
greater than 50 mM. A proliferative effect was noted at expression in prostate cells.38 HSP90 is a chaperone
lower concentrations. Additionally, they observed that protein that aids in the maintenance of oncoproteins,
increased presence of ER-b independently contributed to such as human epidermal growth factor 2 (HER2) and
apoptosis.35 Presumably, this is due to ER-b s apoptotic insulin-like growth factor binding protein-2 (IGFBP-2).
abilities when bound by a ligand and to its ability to One can speculate that decreased HSP90 in the cell would
decrease ER-a s proliferative effect.36,37 ER-b is able to likely lead to diminished oncoprotein functionality and
induce apoptosis by increasing intracellular pH via subsequently decrease cancerous growth. HSP90 expres-
modulation of the cells Na+/H+ exchanger, thus inducing sion is positively correlated with the degree of aggression
Bax and mitochondria-directed apoptosis.36 This ability is of prostate cancer cells and is overexpressed in malignant
increasingly effective when ER-a s proliferative activity is prostate cells.38 Aalinkeel et al.38 found that the levels of
also decreased by ER-b.36 It is via this pathway that quer- HSP90, HER2, and IGFBP-2 were reduced as the concen-
cetin is indirectly able to increase Bax and apoptosis in tration (0 100 mM) of quercetin increased in prostate
some ER-b-positive cell lines. cancer cells (LNCaP and PC-3). Conversely, they found
424 Nutrition Reviews® Vol. 68(7):418 428
Figure 3 Diagram of several pro-apoptotic pathways triggered by quercetin concentrations greater than 40 mM. Quer-
cetin can initiate cell death via induction of ER stress. Quercetin can also modulate HSP activity, which leads to alterations in cell
repair and proliferation. Note: Arrows do not always indicate a direct mechanism of action.
that quercetin treatment in healthy prostate cells did not -3, andthencell death.41 As mentioned previously, Aakin-
show this affect.38 Quercetin s selective effect in this cell keel et al.38 found that quercetin treatment was able to
line may be due to its propensity to target HSP90, which decrease HSP90 levels in the cell. Further, they found that
is abundant in tumor cells but not in healthy cells. This the amount of caspase-9 and caspase-3 activity increases
feature of quercetin action, which has also been seen in in a dose-dependent manner with the concentration of
other cell lines,38 makes it a viable substrate for use in quercetin added (Figure 3).38 Although further research
anti-cancer therapy. needs to be done, it is conceivable that quercetin is able to
induce apoptosis via endoplasmic reticulum stress in
some cell lines.
HSP70 is also involved in alleviating endoplasmic
QUERCETIN AND ENDOPLASMIC RETICULUM STRESS
reticulum stress. As mentioned previously, quercetin
Very little research has been done on quercetin s affect on decreases HSP70 expression in cells.39,40 Recently, MCF-7,
endoplasmic reticulum stress. However, a link can clearly T47D, and MDA-MB-435 cell lines have demonstrated
be made. Recall that the endoplasmic reticulum is the that when HSP70 is inhibited by quercetin treatments of
cellular organelle responsible for the packaging and syn- 100 mM, there is subsequent initiation of the unfolded
thesis of many nutrients, among other functions. Endo- protein response.40 This is problematic since this endo-
plasmic reticulum stress is also known as the unfolded plasmic reticulum stress pathway initiates an increased
protein response, as an accumulation of misshapen pro- expression of glucose-regulated protein 78 (GRP78).40
teins increases endoplasmic reticulum stress in cells.40 GRP78 functions to protect cells against chemotherapy
Heat shock proteins prevent endoplasmic reticulum and to increase cell survival.40,42 This provides a mecha-
stress by catalyzing refolding of proteins in the cell. Spe- nism for cells to resist quercetin-induced apoptosis.
cifically, inhibition of HSP90 has been shown to induce However, it was recently established that when GRP78 is
endoplasmic reticulum stress and the subsequent endo- inhibited, there is increased quercetin-mediated apopto-
plasmic reticulum stress proapoptotic pathways.41 Endo- sis. This provides evidence that quercetin may be able to
plasmic reticulum stress has been proposed to initiate work cooperatively with other compounds in order to
mitochondria-mediated apoptosis by increasing intermi- mediate cell death via the endoplasmic reticulum stress
tochondrial calcium concentration.41 The increased pathway.
calcium concentration leads to increased recruitment of In an indirect fashion, previous discoveries have pro-
Bax,41 decreased mitochondrial membrane potential, and vided researchers with more evidence that quercetin is
subsequently cytochrome C release from the mitochon- able to involve endoplasmic reticulum stress pathways in
dria. This release triggers the activation of caspase-9 and order to decrease cell viability. Eukaryotic initiation
Nutrition Reviews® Vol. 68(7):418 428 425
factor-2 (eIF-2) is responsible for regulating protein syn-
thesis in vivo.43 PKR-like endoplasmic reticulum kinase
(PERK) is a protein located on the endoplasmic reticu-
lum. PERK is responsible for the phosphorylation of the
a-subunit of the eIF-2 and is activated under endoplas-
mic reticulum stress.43 When the eIF-2 s a-subunit is
phosphorylated it is unable to dissociate from another
initiation factor, which essentially prevents mRNA and
protein synthesis.43 Ito et al. observed that a 100 mM quer-
cetin treatment was able to increase eIF-2 a-subunit
phosphorylation and decrease protein synthesis in mul-
tiple mouse and human lymphoma and leukemia cell
lines. Upon further investigation, this group found that
quercetin was able to stimulate PERK activity, along with
two other eIF kinases, in order to catalyze phosphoryla-
tion of eIF-2 and decrease protein synthesis.43 Decreased
protein synthesis generally leads to decreased cell growth,
repair, and viability. It is thought that this is further evi-
dence that quercetin uses endoplasmic reticulum stress as
a mechanism to induce eventual apoptosis and cell cycle
arrest in tumors.43
INTERACTION OF PATHWAYS
Figure 4 Depiction of theoretical quercetin penetration/
accumulation in a carcinogenic tumor. Quercetin is better
Low levels of quercetin could likely be achieved in the
able to accumulate near the well-vascularized areas of a
diet for long periods without supplementation, so inves-
tumor. The poorly vascularized areas of a tumor tend to have
tigating the effects of dietary quercetin on cancer preven-
decreased perfusion resulting in micronutrient deficiencies.
tion is important, while investigating higher levels for
Additionally, in areas of decreased blood supply, there is
therapeutic purposes would likely require supplementa-
increased interstitial tumor pressure and other microenvi-
tion or infusion during a therapy. However, because the
ronmental factors that make delivery of molecules such as
exact cellular concentrations of quercetin and its accu-
quercetin to these tumor regions a challenge.
mulation in cells have yet to be determined, the cellular
concentrations could be higher than currently supposed.
Also, many of the mechanisms and pathways described in
to areas of hypoxia.47 In this case, highly perfused areas of
this review could be minimally activated at what would be
a tumor would likely achieve higher concentrations
considered low-dose exposure and then combined to act
of quercetin compared to poorly perfused regions
synergistically. For example, as discussed above, the mito-
(Figure 4). Therefore, the study of quercetin requires
chondrial and death-domain pathways commonly act
knowledge of the effects of the conjugates, tissue concen-
together to induce apoptosis. Specifically, both TRAIL
trations, and the duration of exposure. Microenviron-
and mitogen-activated protein kinase pathways intersect
mental factors such as perfusion, pH, and hypoxia may
and activate FAS, leading to cell death (Figure 2). In addi-
also play a role.
tion, the inhibition of multiple HSP pathways that con-
verge to protect the cell could also result in a combinatory
effect (Figure 3).
CONCLUSION
Studies on cancer prevention in mice also focus on
factors other than ROS as a mechanism of cancer preven- This review presents some of the most recent data
tion, such as the modulation of various signaling path- regarding the pathways involved in the quercetin
ways. For example, recent studies by Ma et al.,44 Moon response. It is proposed that quercetin could be used in
et al.,45 and Miyamoto et al.46 indicated that quercetin at both the prevention and treatment of cancer and that
dietary concentrations inhibited proliferation and led to diet would likely fulfill the concentration requirements
chemoprevention in mice. Therefore, it is also conceiv- for prevention, but supplementation or another form of
able that pathways initiated by low and high doses could delivery could be necessary for therapeutic responses.
interact for therapeutic purposes. Many tumors outgrow Enzymatic modification of quercetin could further lower
their blood supply and are poorly perfused, giving way the threshold necessary for anti-tumor activity. It cannot
426 Nutrition Reviews® Vol. 68(7):418 428
be ruled out that greater understanding of these path- 6. Alia M, Mateos R, Ramos S, et al. Influence of quercetin and
rutin on growth and antioxidant defense system of a human
ways could lead to the promotion of quercetin in con-
hepatoma cell line HepG2. Eur J Nutr. 2006;45:19 28.
ventional therapies and its use with other drugs in order
7. Walle T, Browning AM, Steed LL, Reed SG, Walle UK. Flavonoid
to interact and produce a therapeutic effect at lower con-
glucosides are hydrolyzed and thus activated in the oral
centrations. Quercetin s ability to interact with electrons
cavity in humans. J Nutr. 2005;135:48 52.
at higher concentrations plays a central role in its 8. Murota K, Terao J. Antioxidative flavonoid quercetin: implica-
tion of its intestinal absorption and metabolism. Arch
mechanism of action, mainly by the activation of pro-
Biochem Biophys. 2003;417:12 17.
teins andDNAdamage leadingtothe inductionof many
9. Day AJ, Gee JM, DuPont MS, Johnson IT, Williamson G.
downstream pathways. Key challenges remain for the
Absorption of quercetin-3-glucoside and quercetin-42 -
study of quercetin, including the determination of quer-
glucoside in the rat small intestine: the role of lactase phlo-
cetin s activity and concentration at the tissue site, the rizin hydrolase and the sodium-dependent glucose
transporter. Biochem Pharmacol. 2003;65:1199 1206.
intracellular concentration achievable, and the effect of
10. Bieger J, Cermak R, Blank R, et al. Tissue distribution of quer-
conjugates on the pathways. Microenvironmental factors
cetin in pigs after long-term dietary supplementation. J Nutr.
may also play a role. Further study is also needed to
2008;138:1417 1420.
explain the observed discrepancies between cancer risk
11. Spencer JP, Kuhnle GG, Williams RJ, Rice-Evans C. Intracellular
and quercetin-containing food intake in larger
metabolism and bioactivity of quercetin and its in vivo
metabolites. Biochem J. 2003;372:173 181.
population-based studies.48 Undoubtedly, measuring any
12. Manach C, Williamson G, Morand C, Scalbert A, Remesy C.
micronutrient in foodstuff is challenging because the
Bioavailability and bioefficacy of polyphenols in humans. I.
amount differs depending on the growth conditions,
Review of 97 bioavailability studies. Am J Clin Nutr.
varietals, and food-preparation methods to name a few.48
2005;81(Suppl):S230 S242.
Studies collecting dietary data are also well characterized
13. Silberberg M, Morand C, Manach C, Scalbert A, Remesy C.
Co-administration of quercetin and catechin in rats alters
to have participant reporting bias that may skew out-
their absorption but not their metabolism. Life Sci.
comes.48 Given the confounding factors of epidemiologi-
2005;77:3156 3167.
cal studies and that basic science research has produced
14. Ferry DR, Smith A, Malkhandi J, et al. Phase I clinical trial of
ample proof that quercetin exerts anti-cancer properties,
the flavonoid quercetin: pharmacokinetics and evidence for
this molecular gem appears worthy of more attention
in vivo tyrosine kinase inhibition. Clin Cancer Res.
and time in the limelight. 1996;2:659 668.
15. Min K, Ebeler SE. Quercetin inhibits hydrogen peroxide-
induced DNA damage and enhances DNA repair in Caco-2
cells. Food Chem Toxicol. 2009;47:2716 2722.
Acknowledgments
16. Boots AW, Kubben N, Haenen GR, Bast A. Oxidized quercetin
reacts with thiols rather than with ascorbate: implication for
Declaration of interest. The authors have no relevant
quercetin supplementation. Biochem Biophys Res Commun.
interests to declare. 2003;308:560 565.
17. Kim GN, Jang HD. Protective mechanism of quercetin and
rutin using glutathione metabolism on HO-induced oxidative
stress in HepG2 cells. Ann N Y Acad Sci. 2009;1171:530 537.
REFERENCES
18. Santos MR, Rodriguez-Gomez MJ, Justino GC, et al. Influence
1. Watjen W, Michels G, Steffan B, et al. Low concentrations of of the metabolic profile on the in vivo antioxidant activity of
flavonoids are protective in rat H4IIE cells whereas high con- quercetin under a low dosage oral regimen in rats. Br J Phar-
centrations cause DNA damage and apoptosis. J Nutr. macol. 2008;153:1750 1761.
2005;135:525 531. 19. Murota K, Hotta A, Ido H, et al. Antioxidant capacity of
2. Awad HM, Boersma MG, Vervoort J, Rietjens IM. Peroxidase- albumin-bound quercetin metabolites after onion consump-
catalyzed formation of quercetin quinone methide- tion in humans. J Med Invest. 2007;54:370 374.
glutathione adducts. Arch Biochem Biophys. 2000;378: 20. Justino GC, Santos MR, Canario S, et al. Plasma quercetin
224 233. metabolites: structure-antioxidant activity relationships.
3. Metodiewa D, Jaiswal AK, Cenas N, Dickancaite E, Arch Biochem Biophys. 2004;432:109 121.
Segura-Aguilar J. Quercetin may act as a cytotoxic prooxidant 21. Thangasamy T, Sittadjody S, Lanza-Jacoby S, et al. Quercetin
after its metabolic activation to semiquinone and quinoidal selectively inhibits bioreduction and enhances apoptosis in
product. Free Radic Biol Med. 1999;26:107 116. melanoma cells that overexpress tyrosinase. Nutr Cancer.
4. Egert S, Wolffram S, Bosy-Westphal A, et al. Daily quercetin 2007;59:258 268.
supplementation dose-dependently increases plasma 22. Thangasamy T, Sittadjody S, Limesand KH, Burd R. Tyrosinase
quercetin concentrations in healthy humans. J Nutr. overexpression promotes ATM-dependent p53 phosphoryla-
2008;138:1615 1621. tion by quercetin and sensitizes melanoma cells to dacarba-
5. Harwood M, Nielewska-Nikiel B, Borzelleca JF, et al. A critical zine. Cell Oncol. 2008;30:371 387.
review of the data related to the safety of quercetin and lack 23. Hao Z, Duncan GS, Chang CC, et al. Specific ablation of the
of evidence of in vivo toxicity, including lack of genotoxic/ apoptotic functions of cytochrome C reveals a differential
carcinogenic properties. Food Chem Toxicol. 2007;45:2179 requirement for cytochrome C and Apaf-1 in apoptosis. Cell.
2205. 2005;121:579 591.
Nutrition Reviews® Vol. 68(7):418 428 427
24. Tan J, Wang B, Zhu L. Regulation of survivin and Bcl-2 in 37. Williams C, Edvardsson K, Lewandowski SA, Strom A,
HepG2 cell apoptosis induced by quercetin. Chem Biodivers. Gustafsson JA. A genome-wide study of the repressive effects
2009;6:1101 1110. of estrogen receptor beta on estrogen receptor alpha signal-
25. Zhang Q, Zhao XH, Wang ZJ. Flavones and flavonols exert ing in breast cancer cells. Oncogene. 2008;27:1019 1032.
cytotoxic effects on a human oesophageal adenocarcinoma 38. Aalinkeel R, Bindukumar B, Reynolds JL, et al. The dietary
cell line OE33 by causing G2/M arrest and inducing apoptosis. bioflavonoid, quercetin, selectively induces apoptosis of
Food Chem Toxicol. 2008;46:2042 2053. prostate cancer cells by down-regulating the expression of
26. Zhang Q, Zhao XH, Wang ZJ. Cytotoxicity of flavones and heat shock protein 90. Prostate. 2008;68:1773 1789.
flavonols to a human esophageal squamous cell carcinoma 39. Wang RE, Kao JL, Hilliard CA, et al. Inhibition of heat shock
cell line KYSE-510 by induction of G2/M arrest and apoptosis. induction of heat shock protein 70 and enhancement of heat
Toxicol In Vitro. 2009;23:797 807. shock protein 27 phosphorylation by quercetin derivatives.
27. Roos WP, Kaina B. DNA damage-induced cell death by apop- J Med Chem. 2009;52:1912 1921.
tosis. Trends Mol Med. 2006;12:440 450. 40. Li M, Wang J, Jin J, et al. Synergistic promotion of breast
28. Chien SY, Wu YC, Chung JG, et al. Quercetin-induced apopto- cancer cells death by targeting molecular chaperone GRP78
sis acts through mitochondrial- and caspase-3-dependent and heat shock protein 70. J Cell Mol Med. 2008;13:4540
pathways in human breast cancer MDA-MB-231 cells. Hum 4550.
Exp Toxicol. 2009;28:493 503. 41. Taiyab A, Sreedhar AS, Rao C. Hsp90 inhibitors, GA and
29. Porte S, Valencia E, Yakovtseva EA, et al. Three-dimensional 17AAG, lead to ER stress-induced apoptosis in rat histiocy-
structure and enzymatic function of proapoptotic human toma. Biochem Pharmacol. 2009;78:142 152.
p53-inducible quinone oxidoreductase PIG3. J Biol Chem. 42. Lee E, Nichols P, Spicer D, et al. GRP78 as a novel predictor of
2009;284:17194 17205. responsiveness to chemotherapy in breast cancer. Cancer
30. Wajant H. The Fas signaling pathway: more than a paradigm. Res. 2006;66:7849 7853.
Science. 2002;296:1635 1636. 43. Ito T, Warnken SP, May WS. Protein synthesis inhibition by
31. Siegelin MD, Reuss DE, Habel A, Rami A, von Deimling A. flavonoids: roles of eukaryotic initiation factor 2alpha
Quercetin promotes degradation of survivin and thereby kinases. Biochem Biophys Res Commun. 1999;265:589 594.
enhances death-receptor-mediated apoptosis in glioma cells. 44. Ma ZS, Huynh TH, Ng CP, et al. Reduction of CWR22 prostate
Neuro Oncol. 2009;11:122 131. tumor xenograft growth by combined tamoxifen-quercetin
32. Deng Y, Lin Y, Wu X. TRAIL-induced apoptosis requires Bax- treatment is associated with inhibition of angiogenesis and
dependent mitochondrial release of Smac/DIABLO. Genes cellular proliferation. Int J Oncol. 2004;24:1297 1304.
Dev. 2002;16:33 45. 45. Moon YJ, Shin BS, An G, Morris ME. Biochanin A inhibits breast
33. Galluzzo P, Martini C, Bulzomi P, et al. Quercetin-induced cancer tumor growth in a murine xenograft model. Pharm
apoptotic cascade in cancer cells: antioxidant versus estrogen Res. 2008;25:2158 2163.
receptor alpha-dependent mechanisms. Mol Nutr Food Res. 46. Miyamoto S, Yasui Y, Ohigashi H, Tanaka T, Murakami A.
2009;53:699 708. Dietary flavonoids suppress azoxymethane-induced colonic
34. Porras A, Zuluaga S, Black E, et al. P38 alpha mitogen- preneoplastic lesions in male C57BL/KsJ-db/db mice. Chem
activated protein kinase sensitizes cells to apoptosis induced Biol Interact. 2009;183:276 283.
by different stimuli. Mol Biol Cell. 2004;15:922 933. 47. Wachsberger PR, Burd R, Marero N, et al. Effect of the tumor
35. Sotoca AM, Ratman D, van der SP, et al. Phytoestrogen- vascular-damaging agent, ZD6126, on the radioresponse of
mediated inhibition of proliferation of the human T47D U87 glioblastoma. Clin Cancer Res. 2005;11:835 842.
breast cancer cells depends on the ERalpha/ERbeta ratio. J 48. Wang L, Lee IM, Zhang SM, et al. Dietary intake of selected
Steroid Biochem Mol Biol. 2008;112:171 178. flavonols, flavones, and flavonoid-rich foods and risk of
36. Subramanian M, Shaha C. Estrogen modulates human cancer in middle-aged and older women. Am J Clin Nutr.
macrophage apoptosis via differential signaling through 2009;89:905 912.
estrogen receptor-alpha and beta. J Cell Mol Med. 2009;13:
2317 2329.
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