Induction of p53 contributes to apoptosis of HCT-116 human colon... cells induced by the dietary compound fisetin

Am J Physiol Gastrointest Liver Physiol 296: G1060–G1068, 2009.
First published March 5, 2009; doi:10.1152/ajpgi.90490.2008.
Induction of p53 contributes to apoptosis of HCT-116 human colon cancer
cells induced by the dietary compound fisetin
Do Y. Lim and Jung Han Yoon Park
Department of Food Science and Nutrition, Hallym University, Chuncheon, Korea
Submitted 12 August 2008; accepted in final form 25 February 2009
Bcl-2 family proteins; permeability of mitochondrial membrane;
caspase; caspase inhibitor; Fas
of the leading causes of death in the
United States (16), and the incidence of this disease is also
increasing rapidly in Asian countries. Thus far, an insufficient
degree of success has been achieved by dietary measures,
chemical compounds, screening and surveillance colonoscopy,
and colonoscopic polypectomy. Thus there is an urgent need to
develop new strategies to reduce the incidence and prevalence
of colorectal cancer. The hallmarks of cancer include tumor
cell proliferation and survival and tumor angiogenesis and
metastasis (11). According to currently available knowledge
regarding the molecular aberrations that underlie carcinogenesis, several promising targets for the prevention and treatment
LARGE BOWEL CANCER IS ONE
Address for reprint requests and other correspondence: J. H. Y. Park, Dept.
of Food Science and Nutrition, Hallym Univ., 39 Hallymdaehak-gil, Chuncheon, 200-702, Korea (e-mail: [email protected]).
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of cancer appear to exist. A common denominator of many of
these targeted strategies is the removal of malignant cells via
the induction of apoptosis while sparing nontransformed cells
(10, 14). Thus bioactive compounds with the ability to induce
apoptosis in cancer cells may potentially be utilized as cancer
chemopreventive and chemotherapeutic agents.
Fisetin, 3,3⬘,4⬘,7-tetrahydroxyflavone, is found in fruits and
vegetables including cucumber, grape, strawberry, apple, persimmon, and onion (1) and was previously shown to evidence
a variety of biological activities, including antioxidant (12),
anti-inflammatory (13, 31), anti-invasive (13), and antiangiogenic (13) effects. In addition, fisetin has been previously
reported to inhibit the proliferation of a variety of cancer cells,
including hepatocarcinoma (20 – 80 ␮mol/l) (2), prostate cancer (IC50, 23– 80 ␮mol/l) (9), and colon cancer (IC50, 55–90
␮mol/l ) (21) cells. Furthermore, it has been reported to induce
apoptosis in SK-HEP-1 hepatocellular carcinoma (2), HL-60
myeloleukemic (22), and LNCaP prostate cancer (19) cells and
has also been demonstrated to enhance tumor necrosis factor
(TNF), doxorubicin, and cisplatin-induced cytotoxicity in
H1299 human lung adenocarcinoma cells (31). However, to the
best of our knowledge, the effects of fisetin on apoptosis in
colon cancer cells and its underlying mechanisms have yet to
be clearly elucidated.
Apoptosis is induced by two major pathways, which are
referred to as the intrinsic and extrinsic pathways. The intrinsic
pathway is induced via alterations in the Bcl-2 family of
proteins, thus increasing the permeability of the mitochondrial
membrane. This increased mitochondrial membrane permeability induces the release of cytochrome c and Smac/Diablo,
which in turn results in the activation of caspase-9. The
extrinsic pathway is mediated by cell surface death receptors,
including Fas, TNF receptors, and receptors (DR4 and DR5)
for TNF-related apoptosis-inducing ligand (TRAIL). Ligand
stimulation of the death receptors induces the formation of a
death-inducing signaling complex (DISC). The final step in
this process is the recruitment and activation of one of the
caspases, typically caspase-8, followed by the activation of
effector caspases, including caspase-3 and caspase-7 (reviewed
in Ref. 17). The two pathways of apoptosis converge on
caspase-3 and subsequently on other proteases and nucleases
that execute the final events of apoptosis.
The p53 tumor suppressor protein performs a critical role in
inducing apoptosis (36). Many of the proapoptotic Bcl-2 family members, including PUMA, Bax, Bik, and Bid, have been
reported to be transcriptional targets of p53 (15, 28, 35). In
addition, p53 is translocated from the cytoplasm to the mitochondria and can directly induce the permeabilization of the
outer mitochondrial membrane via the formation of complexes
with the protective Bcl-xL and Bcl-2 proteins, thus resulting in
cytochrome c release (25). Furthermore, p53 stimulates a
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Lim DY, Park JH. Induction of p53 contributes to apoptosis of
HCT-116 human colon cancer cells induced by the dietary compound
fisetin. Am J Physiol Gastrointest Liver Physiol 296: G1060 –G1068,
2009. First published March 5, 2009; doi:10.1152/ajpgi.90490.2008.—
Fisetin, or 3,3⬘,4⬘,7-tetrahydroxyflavone, is present in fruits and vegetables and has been previously reported to inhibit the proliferation of
a variety of cancer cells (Lu X, Jung J, Cho HJ, Lim do Y, Lee HS,
Chun HS, Kwon DY, Park JH. J Nutr 135: 2884 –2890, 2005). We
have demonstrated in a previous work that 20 – 60 ␮mol/l fisetin
inhibits cyclin-dependent kinase activities resulting in cell cycle arrest
in HT-29 colon cancer cells. In the present study, we attempted to
characterize the mechanisms by which fisetin induces apoptosis in
HCT-116 cells. DNA condensations, cleavage of poly(ADP-ribose)
polymerase (PARP), and cleavage of caspases 9, 7, and 3 were
induced in HCT-116 cells treated with 5–20 ␮mol/l of fisetin. Fisetin
induced a reduction in the protein levels of antiapoptotic Bcl-xL and
Bcl-2 and an increase in the levels of proapoptotic Bak and Bim.
Fisetin did not affect the Bax protein levels, but induced the mitochondrial translocation of this protein. Fisetin also enhanced the
permeability of the mitochondrial membrane and induced the release
of cytochrome c and Smac/Diablo. Additionally, fisetin caused an
increase in the protein levels of cleaved caspase-8, Fas ligand, death
receptor 5, and TNF-related apoptosis-inducing ligand, and the
caspase-8 inhibitor Z-IETD-FMK suppressed fisetin-induced apoptosis and the activation of caspase-3. Furthermore, fisetin increases p53
protein levels, and the inhibition of p53 expression by small interference RNA resulted in a decrease in the fisetin-induced translocation of
Bax to the mitochondria, release of mono- and oligonucleosome in the
cytoplasm, and PARP cleavage. These results show that fisetin induces apoptosis in HCT-116 cells via the activation of the death
receptor- and mitochondrial-dependent pathway and subsequent activation of the caspase cascade. The induction of p53 results in the
translocation of Bax to the mitochondria, which contributes to fisetininduced apoptosis in HCT-116 cells.
FISETIN INDUCES APOPTOSIS OF COLON CANCER CELLS
conformational change in Bax, favoring its migration to the
mitochondria (3, 35).
We have previously demonstrated that fisetin inhibits cell
cycle progression at concentrations of 20 – 60 ␮mol/l in HT-29
colon cancer cells harboring a mutant p53 gene (24). In this
study, we attempted to ascertain whether fisetin induces apoptosis in colon cancer cells, as well as the mechanisms of its
action, using HCT-116 human colon cancer cells harboring the
wild-type p53 gene. We have demonstrated that fisetin induces
apoptosis at concentrations of 5–20 ␮mol/l, via the activation
of both death-receptor- and mitochondrion-dependent pathways. In addition, we have determined that the induction of
p53 contributes to fisetin-induced Bax translocation to the
mitochondria and apoptosis.
Materials. All reagents and chemicals were purchased from the
following suppliers: fisetin, anti-␤-actin, RIA-grade BSA, transferrin,
7-aminoactinomycin D, and Hoechst 33258 (Sigma, St. Louis, MO);
phycoerythrin (PE)-conjugated annexin V (annexin V-PE), and antibodies against caspase-8, cytochrome c, and TRAIL (BD Pharmingen,
Franklin Lakes, NJ); antibodies against cleaved caspase-9, cleaved
caspase-7, cleaved caspase-3, poly (ADP-ribose) polymerase (PARP),
Bcl-xL, Bid, Bik, Bak, Bok, and Bim (Cell Signaling, Beverly, MA);
antibodies against ␣-tubulin, Smac/Diablo, Bcl-2, Bax (N-20), Bax
(B-9) AF488, Fas, Fas ligand (Fas-L), p53, and MDM2, p53 small
interfering RNA (siRNA), control A siRNA, and caspase-8 inhibitor
(Z-IETD-FMK) (Santa Cruz Biotechnology, Santa Cruz, CA); 5,5⬘,6,6⬘tetrachloro-1,1⬘,3,3⬘-tetraethyl-imidacarbocyanine iodide (JC-1), and
MitoTracker Red CMXRos (Invitrogen, Carlsbad, CA); and Cell
Death Detection ELISAPLUS kit (Roche, Mannheim, Germany).
Cell culture. HCT-116, HT-29, and IEC-6 cells were obtained from
the American Type Culture Collection (Manassas, VA) and maintained in DMEM/F12 containing 100 ml/l of FBS with 100,000 U/l of
penicillin and 100 mg/l of streptomycin. HCT-116 cells between
passages 23 and 33 were employed in these studies. To assess the
effects of fisetin, the cells were plated with DMEM/F12 containing
10% FBS. Prior to fisetin treatment, the monolayers were rinsed and
serum starved for 24 h with DMEM/F12 supplemented with 5 mg/l of
transferrin, 0.1 g/l of BSA and 5 ␮g/l of selenium (serum-free
medium). After serum starvation, the medium was replaced with fresh
serum-free medium with or without the indicated concentrations of
fisetin. Viable cell numbers were estimated via MTT assay, as previously described (7). Fisetin is dissolved in DMSO, and all cells were
treated with DMSO at a final concentration of 0.01%.
Hoechst 33258 staining. HCT-116 cells were plated in four-well
chamber slides and treated for 24 h with various concentrations of
fisetin, as described above. The apoptotic morphological changes were
assessed via Hoechst 33258 staining, as previously described (20).
JC-1 staining. To measure the mitochondrial membrane potential,
the cells were stained with the dual emission dye JC-1 (27). The cells
were incubated for 24 h with 0 –20 ␮mol/l of fisetin, loaded with JC-1,
then analyzed via FACScan (Becton Dickinson, Franklin Lakes, NJ),
as previously described (18).
Western blot analysis. The cell lysates were prepared as previously
described (5). Cytosolic and mitochondrial proteins were separated as
previously described (18). The protein contents were determined by
using a BCA protein assay kit (Pierce, Rockford, IL). The proteins
were resolved on SDS-PAGE (4 –20% or 10 –20%) and transferred
onto polyvinylidene fluoride membranes (Millipore, Billerica, MA).
The blots were then blocked for 1 h with 50 g/l of nonfat dry milk
dissolved in 20 mmol/l of Tris 䡠 HCl (pH 7.5), 150 mmol/l NaCl, and
0.1% Tween 20 (TBST), then incubated for 1 h with antibody raised
against cleaved caspase-9 (1:1,000), cleaved caspase-3 (1:1,000),
cleaved caspase-7 (1:1,000), cleaved PARP (1:1,000), ␤-actin (1:
2,000), Smac/Diablo (1:1,000), cytochrome c (1:1,000), Bcl-xL (1:
1,000), Bcl-2 (1:1,000), Bid (1:500), Bax (1:1,000), Bak (1:1,000),
Bok (1:1,000), Bim (1:1,000), caspase-8 (1:1,000), Fas-L (1:1,000),
Fas (1:1,000), DR5 (1:1,000), TRAIL (1:1,000), p53 (1:1,000), or
MDM2 (1:1,000). The blots were subsequently incubated with antimouse, anti-rabbit, or anti-goat horseradish peroxidase-conjugated
antibody. Signals were detected via enhanced chemiluminescence by
using Super-Signal West Dura Extended Duration Substrate (Pierce)
or Immobilon Western HRP Substrate (Millipore). Densitometric
analyses were conducted using the Bio-profile Bio-ID application
(Vilber-Lourmat, France). The expression levels were normalized to
␤-actin or ␣-tubulin, and the control (0 ␮mol/l fisetin) levels were set
to 100%.
Annexin V staining and fluorescence-activated cell sorting analysis. The cells were plated on 12-well plates at 100,000 cells/well in
DMEM-F12 containing 10% FBS. The cells were subjected to serum
starvation and then pretreated for 4 h with 0 or 20 ␮mol/l of caspase-8
inhibitor (Z-IETD-FMK) prior to treatment with 0 or 20 ␮mol/l fisetin
for 24 h. The cells were trypsinized and incubated for 15 min in
darkness with annexin V and 7-aminoactinomycin D at room temperature. Apoptotic cells were analyzed via flow cytometry, by use of a
FACScan system. The data were analyzed by using Modfit version 1.2
software (Verity Software, Topsham, ME).
Suppression of p53 expression by siRNA. To suppress p53 expression with siRNA, 2 ⫻ 106 cells were transfected with 2.3 ␮mol/l of
p53 siRNA or control A siRNA (Santa Cruz Biotechnology) by using
Amaxa Cell Lines Nucleofector kit R (Amaxa, Gaithersburg, MD) in
accordance with the manufacturer’s recommended protocols.
Cell death detection ELISAPLUS assay. Cells transfected with p53
siRNA or control A siRNA were plated in 12-well plates at a density
of 100,000 cells/well and treated for 24 h with 0 or 20 ␮mol/l of
fisetin. The cell lysates were then assayed for the quantitative determination of mono- and oligonucleosomes released into the cytoplasm
by using a Cell Death Detection ELISAPLUS kit (Roche) in accordance with the manufacturer’s recommended protocol.
Immunocytochemistry. HCT-116 cells transfected with p53 siRNA
or control A siRNA were plated on four-well chamber slides and
treated with fisetin as described above. After fisetin treatment, the cell
monolayers were incubated for 40 min with 10 nmol/l of MitoTracker
Red CMXRos in DMEM/F12 supplemented with 5% FBS. The cells
were fixed for 30 min with 4% formaldehyde buffer containing 0.5%
saponin at room temperature, then blocked for 20 min with 10% goat
serum (Santa Cruz Biotechnology) in TBST. Bax (B-9) AF488 antibody (1:100) was applied for 1 h at room temperature, after which the
cell monolayer was washed and mounted with Mounting Solution
(Sigma). Photographs were taken with a Carl Zeiss AxioImager
microscope (Carl Zeiss, Jena, Germany).
Statistical analyses. The results are expressed as means ⫾ SE and
were analyzed via ANOVA. Differences among the treatment groups
were assessed by Duncan’s multiple-range test, using the SAS system
for Windows version 8.1. Differences were considered significant at
P ⬍ 0.05.
RESULTS
Fisetin induces apoptosis of HCT-116 cells. In a previous
work (24), we reported that fisetin dose dependently inhibited
the growth of HT-29 cells harboring a mutant p53 gene (8), at
concentrations of 20 – 60 ␮mol/l. To determine whether fisetin
inhibits the growth of HCT-116 cells containing the wild-type
p53 gene, the cells were treated with various concentrations
(0 –20 ␮mol/l). Fisetin induced a marked dose-dependent reduction in the number of viable HCT-116 cells (Fig. 1A). The
same concentration (20 ␮mol/l) of fisetin caused only a slight
reduction in HT-29 cell growth (24) and exerted no detectable
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MATERIALS AND METHODS
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Fig. 1. Fisetin induces apoptosis and cleavage of caspases in HCT-116 cells. HCT-116 cells were plated on 24-well plates at a density of 50,000 cells/well (A),
in 4-well chamber slides at a density of 50,000 cells/well (C), and in 100-mm dishes at a density of 2,000,000 cells/dish (D) with DMEM/F12 supplemented
with 10% FBS. IEC-6 cells were plated in 24-well plates at a density of 50,000 cells/well (B). At 24 h after plating, the monolayers were serum starved in
DMEM/F12 supplemented with 5 mg/l transferrin, 0.1 g/l BSA, and 5 ␮g/l selenium (serum-free medium) for 24 h and subsequently incubated in serum-free
medium containing 0, 5, 10 or 20 ␮mol/l of fisetin. A and B: cells were treated with fisetin for the indicated times and the cell numbers were estimated via MTT
assay. Each bar represents mean ⫾ SE (n ⫽ 6). Means without a common letter differ, P ⬍ 0.05. C: HCT-116 cells were treated for 24 h with fisetin and stained
with the DNA-specific dye Hoechst 33258. D: HCT-116 cells were treated for 24 h with fisetin and the total cell lysates were subjected to Western blotting with
the indicated antibodies. Photographs of the chemiluminescent detection of the blots, which are representative of 3 independent experiments, are shown. The
relative abundance of each band to its own ␤-actin was quantified, and the control levels were set to 100%. The adjusted mean ⫾ SE (n ⫽ 3) of each band is
shown above each blot. Means without a common letter differ, P ⬍ 0.05.
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FISETIN INDUCES APOPTOSIS OF COLON CANCER CELLS
depolarized mitochondrial membranes, in a concentration-dependent manner (Fig. 2A). Fisetin also increased the levels of
Smac/Diablo and cytochrome c in the cytoplasm and decreased
those levels in the mitochondria (Fig. 2B).
Fisetin alters the levels of Bcl-2 family proteins and induces
Bax translocation to mitochondria. Because fisetin increased
the permeability of the mitochondrial membranes and because
Bcl-2 proteins have been recognized to regulate the permeability of the mitochondrial membrane, we then attempted to
determine whether fisetin alters the levels of the Bcl-2 family
proteins. Fisetin induced a reduction in the levels of Bcl-xL
and Bcl-2 proteins but increased those of Bak, BimEL, BimL,
and BimS. Truncated Bid was not detected, whereas the levels
of intact Bid were decreased in the fisetin-treated cells, thus
suggesting that fisetin may have induced the cleavage of Bid.
An alteration in the size of Bik was detected in the cells treated
with 20 ␮mol/l of fisetin (Fig. 3A). Bax levels remained
unchanged in the fisetin-treated cells (Fig. 3A), but Bax levels
were reduced in the cytoplasm and increased in the mitochondria of the fisetin-treated cells (Fig. 3B).
Fisetin increases cleavage of caspase-8. Because the levels
of intact Bid were reduced and the levels of cleaved caspase-3
were increased in the fisetin-treated cells, we attempted to
determine the effects of fisetin on caspase-8 activation, via
Western blot analysis. Fisetin induced a decrease in the levels
of uncleaved caspase-8 and an increase in the levels of cleaved
Fig. 2. Fisetin induces mitochondrial membrane depolarization and the release of Smac/Diablo and cytochrome c from mitochondria in HCT-116 cells. Cells
were treated with fisetin as described in Fig. 1D.
A: cells were loaded with 5,5⬘,6,6⬘-tetrachloro1,1⬘,3,3⬘-tetraethyl-imidacarbocyanine iodide (JC-1)
and then analyzed via flow cytometry. The number of
cells with normal polarized mitochondrial membranes
(red) and cells with depolarized mitochondrial membranes (green) is expressed as a percentage of the total
cell number. Each bar represents mean ⫾ SE (n ⫽ 6).
Means without a common letter differ, P ⬍ 0.05.
B: cells were subjected to subcellular fractionation.
The resulting cytosolic and mitochondrial fractions
were analyzed via Western blotting with antibodies
raised against Smac/Diablo, cytochrome c, heat shock
protein (HSP)60, or ␣-tubulin. Photographs of the
chemiluminescent detection of the blots, which are
representative of 3 independent experiments, are
shown. The relative abundance of each band to its own
␣-tubulin was quantified, and the control levels were
set to 100%. The adjusted mean ⫾ SE (n ⫽ 3) of each
band is shown above each blot. Means without a
common letter differ, P ⬍ 0.05.
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effects on the viability of IEC-6 normal intestinal epithelial
cells (Fig. 1B).
To determine whether fisetin induces apoptosis in HCT-116
cells, the cells were treated for 24 h with fisetin at 0 –20 ␮mol/l,
then stained with Hoechst 33258 dye. Fisetin increased the
number of cells with condensed and fragmented nuclei, which
is consistent with the morphological features of apoptosis (Fig.
1C). Also in a manner consistent with the morphological
features of apoptosis, fisetin increased the cleavage of PARP,
an enzyme that is crucial to the repair of DNA damage. The
ability of PARP to repair DNA damage is abrogated after
cleavage of PARP, and thus cleaved PARP is generally considered to be a useful marker for apoptosis. Fisetin increased
the cleavage of caspases 9, 3, and 7 in HCT-116 cells (Fig.
1D). In the HT-29 cells, similar effects on cell death, caspase
activation, and PARP cleavage were observed with higher
concentrations (20 – 60 ␮mol/l) of fisetin (data not shown).
Fisetin induces depolarization of the mitochondrial membrane in HCT-116 cells. Since fisetin induced the activation of
caspase-9, we subsequently attempted to determine whether
fisetin increases mitochondrial membrane permeability in
HCT-116 cells. Fisetin-treated cells were stained with JC-1,
and the percentages of cells evidencing green-positive and
red-negative fluorescence were scored as depolarized cells via
FACS analysis. Fisetin reduced the number of cells with
normal mitochondria and increased the number of cells with
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FISETIN INDUCES APOPTOSIS OF COLON CANCER CELLS
Fig. 3. Fisetin alters the levels of Bcl-2 family proteins in HCT-116 cells.
Cells were treated with fisetin as described in Fig. 1D. A: cell lysates were
subjected to Western blotting with indicated antibodies. B: cells were subjected
to subcellular fractionation as described in Fig. 2 and then analyzed via
Western blotting with Bax antibody. Photographs of the chemiluminescent
detection of the blots, which are representative of 3 independent experiments,
are shown. The relative abundance of each band to its own ␤-actin (A) or
␣-tubulin (B) was quantified, and the control levels were set at 100%. The
adjusted mean ⫾ SE (n ⫽ 3) of each band is shown above each blot. Means
without a common letter differ, P ⬍ 0.05.
Fig. 4. Fisetin increases the levels of cleaved caspase-8, Fas ligand (Fas-L),
TRAIL, and DR5 in HCT-116 cells. Cells were treated with fisetin as described
in Fig. 1D. Cell lysates were subjected to Western blotting with indicated
antibodies. Photographs of the chemiluminescent detection of the blots, which
are representative of 3 independent experiments, are shown. The relative
abundance of each band to its own ␤-actin was quantified, and the control
levels were set to 100%. The adjusted mean ⫾ SE (n ⫽ 3) of each band is
shown above each blot. Means without a common letter differ, P ⬍ 0.05.
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caspase-8. In addition, the levels of Fas-L, TRAIL, and DR5
increased in the fisetin-treated cells, but the levels of Fas
remained unaltered (Fig. 4).
A caspase-8 inhibitor mitigates fisetin-induced apoptosis.
Since caspase-8 was activated in the fisetin-treated HCT-116
cells, we attempted to determine whether the inhibition of
caspase-8 would mitigate the fisetin-induced apoptosis. The
pretreatment of cells with the caspase-8 inhibitor Z-IETDFMK prior to fisetin treatment resulted in an increase in the
number of intact cells and a reduction in the numbers of
apoptotic cells, compared with what was observed with the
cells treated only with fisetin (Fig. 5A). The fisetin-induced
reduction in intact Bid levels and increase in caspase-3 cleavage were prevented in the presence of the caspase-8 inhibitor.
The fisetin-induced cleavage of caspase-9 was suppressed
slightly when the cells were pretreated with the caspase-8
inhibitor (Fig. 5B).
Inhibition of p53 expression results in the mitigation of
fisetin-induced apoptosis and the translocation of Bax to the
mitochondria. The induction of the p53 tumor suppressor
protein can initiate either cell cycle arrest and DNA repair or
FISETIN INDUCES APOPTOSIS OF COLON CANCER CELLS
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Immunocytochemistry was conducted to visualize the cellular
distribution of Bax with a confocal microscope. Without fisetin, Bax was distributed diffusely throughout the cytosol and
mitochondria. After fisetin treatment, Bax clustered and colocalized with mitochondria, which is shown as a bright yellow
coloration in the merged image. This effect was blocked via
transfection with p53 siRNA (Fig. 6E).
DISCUSSION
apoptosis. The p53 tumor suppressor protein evidenced markedly increased levels in cells treated with 20 ␮mol/l of fisetin,
whereas the amount of its negative regulator, oncoprotein
MDM2 (30), was decreased in these cells (Fig. 6A). Western
blotting showed that the transfection of cells with p53 siRNA
significantly abrogated fisetin-induced increases in the levels of
p53 and cleaved PARP (Fig. 6B). Fisetin increased the amount
of mono- and oligonucleosomes released into the cytoplasm,
which was suppressed in p53 siRNA-transfected HCT-116
cells (Fig. 6C), indicating that the induction of p53 contributes
to fisetin-induced apoptosis in these cells.
The p53 siRNA-transfected cells were subjected to subcellular fractionation, and the mitochondrial fractions were evaluated via Western blotting. Bax levels were significantly lower
in the mitochondria of fisetin-treated cells transfected with p53
siRNA compared with cells treated only with fisetin (Fig. 6D).
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Fig. 5. The caspase-8 inhibitor Z-IETD-FMK mitigates fisetin-induced apoptosis in HCT-116 cells. Cells were plated in 12-well plates at a density of
100,000 cells/well (A) and in 100 mm dishes at a density of 2,000,000
cells/dish (B), as described in Fig. 1. After serum starvation, cells were treated
for 4 h with 0 or 20 ␮mol/l of Z-IETD-FMK. Fisetin (0 or 20 ␮mol/l) was then
added and the incubation was continued for another 24 h. A: after the 24
h-incubation, the cells were trypsinized, loaded with 7-aminoactinomycin D
and annexin V, then analyzed via flow cytometry. The number of living cells
and early apoptotic cells is expressed as a percentage of the total cell number.
Each bar represents mean ⫾ SE (n ⫽ 6). Means without a common letter
differ, P ⬍ 0.05. B: total cell lysates were subjected to Western blotting with
the indicated antibodies. The relative abundance of each band to its own
␤-actin was quantified, and the control levels were set to 100%. The adjusted
mean ⫾ SE (n ⫽ 3) of each band is shown above each blot. Means without a
common letter differ, P ⬍ 0.05.
The hallmarks of cancer include tumor cell proliferation and
survival, as well as tumor angiogenesis and metastasis (11). In
a previous study, we demonstrated that fisetin inhibits CDK
activities, resulting in cell cycle arrest in HT-29 colon cancer
cells (24) harboring a mutant p53 gene. Sung et al. (31)
reported that fisetin downregulates nuclear factor-␬B-regulated
gene products associated with cell proliferation, antiapoptosis,
and metastasis in human lung cancer cell lines. A more current
report demonstrated that fisetin induces apoptosis and cell
cycle arrest in LNCaP human prostate cancer cells (19). In this
study, we have demonstrated that fisetin induces apoptosis in
HCT-116 human colon cancer cells harboring the wild-type
p53 gene and that the induction of p53 contributes to fisetininduced apoptosis. Our results demonstrated that fisetin induced nuclear condensation, the cleavage of caspases 9, 3, and
7, and the cleavage of PARP (Fig. 1) at low concentrations
(5–20 ␮mol/l) relative to those utilized in other studies on the
subject. In vivo animal model tests to determine whether fisetin
induces apoptosis in colon cancer cells have yet to be conducted.
Caspases are a family of proteases that are collectively
known as one of the principal executioners of apoptosis, and
their cleavage and subsequent activation are considered the
primary hallmarks of apoptosis. In the death receptor-dependent pathway, death receptors and their ligands (e.g., Fas/Fas-L
and DR5/TRAIL) induce caspase-8 activation; activated
caspase-8 in turn cleaves Bid and/or directly triggers the
activation of caspase-3. When cleaved, truncated Bid migrates
to the mitochondria, where it increases the permeability of the
mitochondrial membrane and induces cytochrome c release
and caspase-9 cleavage (17). Our investigations have revealed
that fisetin treatment induced an increase in the levels of
cleaved caspase-8. The protein levels of Fas-L, DR5, and
TRAIL were also shown to have increased in the fisetin-treated
cells (Fig. 4). Furthermore, the caspase-8 inhibitor, Z-IETDFMK, mitigated fisetin-induced apoptosis, the cleavage of
caspases-3 and -9, and an increase in intact Bid levels (Fig. 5).
These findings show that the activation of the death receptor
pathway is one of the mechanisms by which fisetin induces
apoptosis in HCT-116 cells.
Caspase-9 is activated principally by the mitochondriondependent pathway, in which apoptotic stimuli increase the
permeability of mitochondrial membranes and the subsequent
release of cytochrome c into the cytoplasm. The released
cytochrome c then binds with procaspase-9 and apoptosis
protease-activating factor 1 (Apaf-1) to form an apoptosome,
in which procaspase-9 is activated to its active form. Activated
caspase-9, in turn, causes the subsequent cleavage of caspase-3
(23). In the present study, fisetin treatment induced the cleavage of caspases 9, 3, and 7 (Fig. 1). Additionally, cytochrome
c was released into the cytoplasm and the depolarization of the
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Fig. 6. Inhibition of p53 induction results in a decrease in fisetin-induced apoptosis. A: cells were treated with fisetin as described in Fig. 1. Total cell lysates
were subjected to Western blotting with antibodies raised against p53, MDM2, or ␤-actin. B–E: cells were transfected with p53 small interfering RNA (siRNA)
or control A siRNA. B: transfected cells were treated with 0 or 20 ␮mol/l fisetin. The total cell lysates were subjected to Western blotting with antibodies raised
against p53, cleaved poly(ADP-ribose) polymerase (PARP), or ␤-actin. C: transfected cells were plated in 12-well plates at a density of 100,000 cells/well and
treated with 0 or 20 ␮mol/l of fisetin. The cell lysates were assayed using Cell Death Detection ELISAPLUS kit to detect apoptotic cells. The mono- and
oligonucleosomes released into the cytoplasm were quantified, and enrichment factor was calculated. Enrichment factor ⫽ absorbance of the sample/absorbance
of the control (control A siRNA transfected, 0 ␮mol/l fisetin). Each bar represents mean ⫾ SE (n ⫽ 12). Means without a common letter differ, P ⬍ 0.05.
D: transfected cells were treated with fisetin and subjected to subcellular fractionation as described in Fig. 2B. The resulting mitochondrial fractions were analyzed
via Western blotting with Bax antibody. The relative abundance of each band to its ␤-actin (A and B) or ␣-tubulin (D) was quantified, and the control levels were
set to 100%. The adjusted mean ⫾ SE (n ⫽ 3) of each band is shown above each blot. Means without a common letter differ, P ⬍ 0.05. E: transfected cells
were treated with fisetin and subjected to immunocytochemistry with Bax-AF488 antibody. Mitochondria were stained with Mitotracker Red (red color) and Bax
probed with anti-Bax-AF488 antibody (green color). Bright yellow coloration of the merged image represents Bax translocated from the cytosol to the
mitochondria.
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FISETIN INDUCES APOPTOSIS OF COLON CANCER CELLS
35
S. Comparison with Bik labeled with 32P revealed that the
slower moving band might be phosphorylated and that this
phosphorylation may have increased the potency of Bik (33).
In our study, the treatment of cells with fisetin induced a more
rapidly migrating band, suggesting that the change in the size
of Bik may not be associated with the induction of apoptosis in
fisetin-treated cells. Further study will be required to determine
the nature of the change in the size of Bik, and whether this is
associated with the activation of Bik.
Fisetin may not be absorbed well and, even after being
absorbed, it may not exist as the parent compound, but rather
as metabolites such as glucuronides, sulfates, or other conjugate species. To date, the absorption, metabolism, blood concentrations, half-life, and biological activity of fisetin have not
been well characterized, except that one study has reported that
fisetin is methylated in the human liver (6). The present study
utilized fisetin at concentrations of 5–20 ␮mol/l, and other
investigators have assessed the effects of fisetin at higher
concentrations than those utilized in our studies. At the present
time, we do not know whether fisetin can be delivered effectively to the target tissue, so that the concentrations of fisetin in
the target tissue as high as those used in the present cell culture
studies can be achieved. However, if fisetin is not absorbed
well, it remains possible that colonocytes will be exposed to
high concentrations of unabsorbed fisetin in the lumen. In an
effort to determine whether the concentrations used in the cell
culture studies are physiologically relevant, the concentrations
of fisetin and its metabolites in human serum, as well as in the
colonic lumen, should be determined in the future.
In conclusion, we have demonstrated that fisetin induces
apoptosis via the activation of caspases in HCT-116 human
colon cancer cells. Our results reveal that fisetin alters the
expression of several Bcl-2 family member proteins and death
receptor signals, ultimately resulting in the induction of the
caspase-9/-3 cascade. Fisetin also causes an increase in the
protein levels of the p53 tumor suppressor, which stimulates
the migration of Bax to the mitochondria. In the present study,
we provide some of the molecular basis for using fisetin as a
potential apoptosis stimulatory agent. Further in vivo evaluations of the potential of fisetin as an antitumorigenic agent are
clearly warranted.
GRANTS
This study was supported by a Korea Science and Engineering Foundation
(KOSEF) grant funded by the Korean government (Ministry of Education,
Science and Technology) (R01-2007-000-20164-0) and a grant from the SRC
program of KOSEF (R11-2008-0106-01003-0).
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