Cucumber & Apigenin
Apigenin in cancer therapy: anti-cancer effects and mechanisms of action
Cucumbers have not received as much press as other vegetables in terms of health benefits, but this widely cultivated food provides us with a unique combination of nutrients. At the top of the phytonutrient list for cucumbers are its cucurbitacins, lignans, and flavonoids. These three types of phytonutrients found in cucumbers provide us with valuable antioxidant, anti-inflammatory, and anti-cancer benefits.
Specific phytonutrients provided by cucumbers include
Flavonoids
- apigenin
- a luleolin
- a quercetin
- a kaempferol
Lignans
- pinoresinol
- lariciresinol
- secoisolariciresinol
Triterpenes
- cucurbitacin A
- cucurbitacin B
- cucurbitacin C
- cucurbitacin D
Apigenin anti-cancer effects and mechanisms of
action
2017 Research
Background
Cancer is a disease
caused by the abnormal proliferation and differentiation of cells and is
governed by tumorigenic factors. Cancer is the second most common cause of
human death worldwide. Currently, chemotherapy is still one of the best
therapeutic methods to treat cancer. With wider application and further
understanding, the side effects and acquired drug resistance of synthesized
small molecule compounds have caused more and more concerns [1, 2]. Therefore, natural and edible small molecules
such as flavones, which are thought to have remarkable physiological effects,
low toxicity and non-mutagenic properties in the human body, have gained more
and more interest in anti-cancer agent development.
Apigenin, known chemically as
4′,5,7-trihydroxyflavone, belongs to the flavone subclass and is abundant in
vegetables, fruits and beverages, such as parsley, grapes, apples, chamomile
tea and red wine. Apigenin is also one of the active ingredients in Chinese
medicinal herbs. In its natural form, apigenin is usually conjugated to a
glycoside in vivo. Apigenin was classified as a class II drug of
Biopharmaceutical Classification System in a recent study [3]. It has a poor solubility in aqueous phase but
high intestinal permeability determined by single-pass intestinal perfusion
technique. For in vivo studies, oral administration of apigenin at
60 mg/kg in rat resulted in low blood levels, with a Cmax of
1.33 ± 0.24 μg/mL and AUC 0–t° of
11.76 ± 1.52 μg h/mL. With novel carbon nanopowder drug
carrier system of solid dispersions, the relative oral bioavailability of
apigenin was enhanced by approximately 183% [4]. To better understand the pharmacokinetics and
distribution of apigenin in vivo, Wistar rats were treated once with
radiolabeled apigenin by oral administration. Then the potential storage tissue
and blood kinetic were analyzed. Results showed that 51.0% of radioactivity was
recovered in urine, 12.0% in feces, 1.2% in the blood, 0.4% in the kidneys,
9.4% in the intestine, 1.2% in the liver, and 24.8% in the rest of the body
within 10 days. Furthermore, blood kinetics analysis indicated that
radioactivity appeared at 9 h and reached a maximum at 24 h
post-ingestion time, suggesting a slow distribution phase and slow elimination.
Thus a possible accumulation of apigenin in the body is hypothesized [5].
Apigenin has been used as a
traditional medicine for centuries because of its physiological functions as an
antioxidant and anti-inflammatory [6, 7], its role in lowering blood pressure [8], and its antibacterial and antiviral properties [9]. In addition to those effects, apigenin was proven
to have tumor suppression efficacy in the last few decades (Fig. 1). Since Birt
et al. first reported that apigenin had anti-cancer activities in 1986 [10], more and more evidence has been presented to
demonstrate that apigenin shows antitumor efficacy against various types of
cancer with both cell lines in vitro and mouse models in vivo.
Molecular structure and physiological functions of
apigenin
Apigenin has been demonstrated
to show broad anti-cancer effects in various types of cancers, including
colorectal cancer, breast cancer, liver cancer, lung cancer, melanoma, prostate
cancer and osteosarcoma [11–16]. This flavone inhibits cancer cell proliferation
by triggering cell apoptosis, inducing autophagy and modulating the cell cycle.
Apigenin also decreases cancer cell motility and inhibits cancer cell migration
and invasion. Recently, apigenin was reported to show anti-cancer activities by
stimulating an immune response [17]. During those processes, multiple signaling
pathways and protein kinases are modulated by apigenin, including PI3K/AKT,
MAPK/ERK, JAK/STAT, NF-κB and Wnt/β-catenin.
In this review, we
focus on apigenin’s antitumor effects and summarize the advancements in the
anti-cancer effects of apigenin and its multiple underlying mechanisms that have
been identified in recent years. We also discuss combinatorial strategies to
enhance the anti-cancer effect of apigenin on various cancers using in vitro
and in vivo models. Our purpose is to highlight apigenin as a promising agent
for cancer therapy.
Apigenin in cancer therapy
Carcinogenesis is a
multistage process and involves a series of genetic and epigenetic changes that
lead to the initiation, promotion and progression of cancer. The strategies to
treat cancer are to eliminate tumor cells by triggering cell apoptosis or to
inhibit cancer cell proliferation by inducing cell cycle arrest, thereby making
cancer a chronic disease and prolonging the survival of patients. Current
strategies include the induction of apoptosis or autophagy, regulation of the
cell cycle, inhibition of tumor cell migration and invasion, and stimulation of
the immune response of patients. Thus far, apigenin has demonstrated all these
antitumor activities with different tumor types in vitro and in vivo. Those
anti-cancer effects of apigenin and the underlying signaling pathways involved
are summarized, as in Fig. 2 and
Table 1.
Apigenin triggers cell apoptosis, autophagy and immune
response and inhibits cell cycle progress and cell migration and invasion by
targeting multiple signaling pathways. Bold arrows of ↙ represents induction
and ⊥ represents
suppression of effects. Light arrow ↑ represents upregulation and ↓ represents
downregulation of molecules pathways
Table 1
Effects of apigenin treatment
alone on cancer cells
Tumor type
|
Cell lines (concentration)
|
Mice (dosages)
|
Therapeutic effects
|
Mechanisms
|
Citations
|
Colorectal cancer
|
SW480 (40 μM)
|
Inhibited proliferation, invasion and migration
|
Inhibited Wnt/β-catenin signaling
|
[11]
|
|
HCT116 (25 μM)
|
Inhibited proliferation; autophagy; apoptosis
|
Suppressed the expression of cyclin B1, Cdc2 and
Cdc25c; induced PARP cleavage; induced LC3-II
|
[28]
|
||
DLD1 and SW480 (40 μM)
|
20 mg/kg (athymic nude mice, intraperitoneally)
|
Inhibited proliferation, invasion and migration
|
Attenuated NEDD9; reduced phosphorylations of FAK,
Src, and Akt
|
[48]
|
|
SW480, DLD-1, and LS174T (40 μM)
|
50 mg/kg (BALB/c-nude mice, orthotopically
implanted)
|
Inhibited proliferation, invasion and migration
|
Up-regulated TAGLN; down-regulated MMP-9 expression;
decreasing phosphorylation of Akt
|
[50]
|
|
Breast cancer
|
BT-474 (40 μM)
|
Inhibited cell proliferation; apoptosis
|
Reduced the p-JAK1, p-JAK2 and p-STAT3; up-regulated
the levels of cleaved caspase-8, cleaved caspase-3 and the cleavage of PARP
|
[25]
|
|
MDA-MD-231 (40 μM)
|
5, 25 mg/kg (BALB/c-nude mice, orthotopically
injected)
|
Cell cycle arrest
|
Suppressed cyclin A, cyclin B, and CDK1; upregulated
p21WAF1/CIP1; inhibited HDAC activity; induced histone H3
acetylation
|
[29]
|
|
MDA-MB-231 and T47D (40 μM)
|
Inhibited cell proliferation; apoptosis
|
Increased levels of caspase3, PARP cleavage and
Bax/Bcl-2 ratios
|
[41]
|
||
MDA-MB-468 and 4T1 (30 μM)
|
Enhanced the immune responses
|
Inhibited IFN-γ-induced PD-L1 expression; inhibited
STAT1
|
[58]
|
||
SKBR3 (40 μM)
|
Apoptosis
|
Reduced the expression of p-JAK2 and p-STAT3;
inhibited VEGF
|
[98]
|
||
MDA-MB-453 (60 μM)
|
Inhibited cell proliferation; apoptosis
|
Up-regulated caspase-8, caspase-3 and the cleavage
of PARP; inactivation of JAK2 and STAT3
|
[99]
|
||
Lung cancer
|
H1299 and H460 (20 μM)
|
Inhibited cell proliferation; apoptosis
|
Suppressed GLUT1
|
[13]
|
|
A549 (40 μM)
|
Inhibited cell proliferation, migration, invasion
|
Decreased the PI3K/Akt signaling pathway
|
[47]
|
||
Prostate cancer
|
LNCaP (20 μM)
|
Inhibited cell proliferation; apoptosis
|
Decreased cyclin D1, D2 and E; upregulated WAF1/p21
|
[15]
|
|
PC-3 and DU145 (20 μM)
|
20, 50 μg/mouse/day (athymic nude mice, oral
gavage)
|
Cell cycle arret; apoptosis
|
Suppression of XIAP, c-IAP1, c-IAP2 and survivin;
decreased Bcl-xL and Bcl-2 and increase in Bax protein
|
[22]
|
|
DU145 (20 μM)
|
Inhibited migration and invasion; cell cycle arrest
|
Increased E-cadherin; decreased snail and vimentin
|
[46]
|
||
20 and 50 μg/mouse/day (TRAMP mice, oral
gavage)
|
Inhibited tumorigenesis
|
Inhibited IKK activation and restored the expression
of IκBα
|
[89]
|
||
PC-3 and 22Rv1 (20 μM)
|
20 and 50 μg/mouse/day (athymic nude mice, oral
gavage)
|
Inhibited cell proliferation, invasivion
|
Inactivation of IKKα; suppressed NF-ĸB/p65
activation
|
[90]
|
|
PC3-M and LNCaP C4-2B (25 μM)
|
Inhibited cell proliferation and metastases
|
Inhibited the Smad2/3 and Src/FAK/ Akt pathways
|
[110]
|
||
PC3 (25 μM)
|
Apoptosis; cell cycle arrest; suppressed stem cell
migration
|
Increased p21 and p27; upregulated caspases-8, -3
and TNF-α; downregulation of PI3K/Akt and NF-κB signaling
|
[65]
|
||
Melanoma
|
A375, C8161 (40 μM)
|
Inhibited proliferation and invasion; apoptosis;
cell cycle arrest
|
Activation of cleaved caspase-3 and cleaved PARP;
decreased ERK1/2 proteins, p-AKT and p-mTOR
|
[14]
|
|
A2058, A375 (20 μM)
|
Inhibited metastasis
|
Inhibited the phosphorylation of FAK/ERK1/2
|
[82]
|
||
A375, G361 (20 μM)
|
150 mg/kg (C57BL/6 mice, oral gavage)
|
Inhibited metastasis
|
Suppressed STAT3 phosphorylation; down-regulated
MMP-2, MMP-9, VEGF and Twist1
|
[97]
|
|
Leukemia
|
HL60 (60 μM)
|
Apoptosis
|
Activation of caspase-9 and caspase-3
|
[23]
|
|
HL60 (50 μM); TF1 (30 μM)
|
Cell cycle arrest
|
Inhibited JAK/STAT pathway
|
[37]
|
||
U937 (40 μM)
|
20, 40 mg/kg (athymic nude mice,
intraperitoneally)
|
Apoptosis
|
Inactivation of Akt; activation of JNK;
downregulated Mcl-1 and Bcl-2
|
[112]
|
|
Ovarian cancer
|
A2780 (20, 40 μM)
|
5 mg/kg (BALB/c nude mice, intraperitoneally)
|
Inhibited adhesion, migration and invasion
|
Inhibited FAK expression
|
[49]
|
SKOV3 (20, 40 μM)
|
Inhibited the self-renewal capacity
|
Downregulated Gli1; inhibition of CK2α
|
[66]
|
||
Glioblastoma
|
GL-15 (50 μM)
|
Inhibited angiogenic
|
Reduced TGF-b1 production
|
[111]
|
|
U87MG and U373MG (25 μM)
|
Inhibited self-renewal capacity
|
Blocked the activation of c-Met signaling
|
[64]
|
||
Renal cell carcinoma
|
ACHN, 786-0, and Caki-1 (20 μM)
|
30 mg/kg (BALB/c-nude mice intraperitoneally)
|
Cell cycle arrest
|
p53 accumulation; modulated ATM signalling
|
[30]
|
Adenoid cystic carcinoma
|
ACC-2 (40 μM)
|
Inhibited proliferation; apoptosis
|
Suppressed the expression of GLUT-1
|
[31]
|
|
Papillary thyroid carcinoma
|
BCPAP (25 μM)
|
Cell cycle arrest; autophagy
|
Down-regulation of Cdc25C expression
|
[32]
|
|
Oral squamous cell carcinoma
|
SCC-25, HaCaT (100 μM)
|
Inhibited proliferation; apoptosis
|
Decreased expression of cyclin D-1 and E;
inactivation of CDK1
|
[33]
|
|
Pancreatic cancer
|
Murine Panc02 (20 μM)
|
25 mg/kg (female C57BL/6N mice, intraperitoneally)
|
Maintain T cell homeostasis
|
Stabilizing Ikaros expression
|
[60]
|
Mesothelioma
|
Malignant mesothelioma (MM) cells (50 μM)
|
20 mg/kg (C57BL/6 mice, oral gavage)
|
Apoptosis
|
Inhibited AKT and c-Jun phosphorylation, and
inhibited NF-κB nuclear translocation
|
[91]
|
Osteosarcoma
|
U2OS and MG63 (50 μg/ml)
|
Inhibited proliferation and invasion
|
Inactivated Wnt/β-catenin signaling
|
[108]
|
|
Head and neck squamous cell carcinoma
|
HSC-3, HN-8, and HN- 30 (40 μM)
|
Suppressed cancer stem cell marker expression
|
Downregulated the stem cell markers of CD44,NANOG,
and CD105, and abolished the hypoxia-induced increase
|
[63]
|
|
Cervical cancer
|
HeLa (40 μM)
|
Inhibited cell self-renewal capacity
|
Downregulation of CK2α expression
|
[67]
|
Induction of
apoptosis
Apoptosis is the
process of programmed cell death. Apoptosis involves energy-dependent cascade
events and different distinct morphological characteristics [18]. To date, apoptosis is induced by two core
pathways: the extrinsic (death receptor) pathway and the intrinsic
(mitochondrial) pathway. Apoptosis is a critical process that allows
undesirable cells to be removed under physiological conditions. Avoiding
apoptosis is one of the most important characteristics of cancer cells that
makes them different from normal cells. Thus, triggering cancer cell apoptosis
by targeting apoptotic pathways with chemotherapy reagents is a widely used
strategy to treat cancer. Apigenin has been demonstrated to be an effective
agent for triggering apoptosis via either the intrinsic or extrinsic pathway in
human cancer cells.
The intrinsic apoptotic pathway
is regulated by the Bcl-2 family of proteins, such as Bcl-2, Bcl-xL, Bcl-w and
Mcl-1, which block apoptosis, while Bad, Bak, Bax, Bid and Bim trigger
apoptosis [19–21]. Apigenin functions to upregulate pro-apoptotic
proteins and/or downregulate pro-survival members, thereby inducing the
intrinsic apoptotic pathway. In prostate cancer therapy, treatment of the
androgen-refractory human prostate cancer cell lines PC-3 and DU145 with
apigenin resulted in apoptosis and a reduction in cell viability caused by a
decrease in Bcl-2 and Bcl-xL and an increase in the active form of the Bax protein,
accompanied by dose-dependent suppression of XIAP, c-IAP1, c-IAP2 and survivin
proteins [22]. In addition, in human promyelocytic leukemia
HL-60 cells, apigenin reduced the mitochondrial outer membrane potential,
released cytochrome c from the mitochondria into the cytosol, induced
procaspase-9 processing, and finally induced cell apoptosis through the
intrinsic apoptotic pathway [23]. In other reports, apigenin caused apoptosis by
changing the ratio of pro-apoptotic to pro-survival mitochondrial proteins.
Apigenin increased the Bax/Bcl-2 ratio in favor of cell apoptosis in prostate
cancer cells [15]. Clearly, apigenin alone is able to trigger
mitochondria-dependent apoptosis in various types of cancer cells.
Moreover, apigenin can enhance
chemotherapy-induced cell apoptosis by modulating the expression level of
mitochondrial proteins. In the colorectal cancer cell lines HCT116 and DLD1,
apigenin upregulated Bim expression and downregulated Mcl-1 expression, thereby
synergizing with the Bcl-2 inhibitor ABT-263 to trigger mitochondria-dependent
cell apoptosis [24].
In addition to
cases where apigenin triggered the intrinsic apoptotic pathway, apigenin was
found to induce cell apoptosis via the extrinsic pathway or both the extrinsic
and intrinsic pathways. Seo et al. found that apigenin neither affected the
levels of Bcl-2 and Bax nor decreased the mitochondrial membrane potential in
the human breast cancer BT-474 cells, but this compound induced extrinsic,
caspase-dependent apoptosis by upregulating the levels of cleaved caspase-8 and
cleaved caspase-3 [25]. In non-small cell lung cancer (NSCLC) cells,
Chen et al. showed that apigenin upregulated the levels of death receptor 4
(DR4) and death receptor 5 (DR5) in a p53-dependent manner, thereby sensitizing
NSCLC cells to TRAIL-induced apoptosis. Meanwhile, apigenin triggered the
intrinsic apoptotic pathway by upregulating the pro-apoptotic proteins Bad and
Bax and downregulating the anti-apoptotic proteins Bcl-xL and Bcl-2 [26]. Moreover, in human keratinocytes and organotypic
keratinocytes, apigenin increased UVB-induced apoptosis via both the intrinsic
and extrinsic apoptotic pathways as well. Apigenin caused changes in Bax
localization and in the release of cytochrome c. Overexpression of the
pro-survival protein Bcl-2 and the dominant-negative form of Fas-associated
death domain protected against apigenin-induced apoptosis [27].
Modulation
of the cell cycle
Uncontrolled and
rapid cell division is another hallmark of cancer. A number of natural
compounds that induce cell cycle arrest have been proved effective for
suppressing cancers in vitro, in vivo and in clinical settings. As evidenced,
apigenin inhibits cancer cell proliferation by modulating the cell cycle and
blocking the cell phase at the G2/M or G0/G1 checkpoint.
In human colorectal carcinoma
HCT116 cells, apigenin treatment potently inhibited cell growth by inducing
cell arrest at G2/M phase, associated with suppression of both cyclin B1 and
its activating partners, Cdc2 and Cdc25c, and increase of cell cycle
inhibitors, p53 and p21WAF1/CIP1 [28]. As in the human breast cancer cell line
MDA-MB-231, Western blotting showed that the expression of cyclin A, cyclin B,
and cyclin-dependent kinase-1 (CDK1) was suppressed by apigenin treatment. In
addition, apigenin upregulated p21WAF1/CIP1 and
increased the interaction of p21WAF1/CIP1 with
proliferating cell nuclear antigen (PCNA), which inhibits cell cycle
progression at the G2/M stage [29]. In addition, in renal cell carcinoma cells,
apigenin caused DNA damage in ACHN cells in a time- and dose-dependent manner
and induced G2/M phase cell cycle arrest through ataxia telangiectasia mutated
(ATM) signal modulation [30]. In adenoid cystic carcinoma (ACC), apigenin
induced G2/M-phase arrest and inhibited ACC-2 cell growth and proliferation in
a dose- and time-dependent manner by decreasing the expression of Glucose
transporter-1 (GLUT-1) [31]. And in human papillary thyroid carcinoma BCPAP
cells, apigenin treatment caused G2/M cell cycle arrest via down-regulation of
Cdc25c expression and stimulated the accumulation of reactive oxygen species
(ROS) production, leading to induction of DNA damage [32].
Moreover, apigenin
can induce cell cycle arrest at the G0/G1 or S checkpoints as well. In human
prostate cancer LNCaP cells, apigenin resulted in G1 arrest of cell cycle
progression. Apigenin treatment markedly decreased the protein expression of
cyclin D1, D2 and E and their activating partners CDK2, 4 and 6, and increased
the expression of p21WAF1/CIP1 and p27KIP1 concomitantly.
The induction of p21WAF1/CIP1appears to be transcriptionally upregulated and p53
dependent [15]. And in an oral squamous cell carcinoma cell line
SCC-25, apigenin treatment caused cell cycle arrest at both G0/G1 and G2/M checkpoints,
associated with decreased expression of cyclin D1 and E, and inactivation of
CDK1 [33]. Interestingly, Solmaz et al. reported that
apigenin exposure induced G2/M arrest in imatinib-sensitive K562 cells while
arresting imatinib-resistant K562/IMA3 cells in S phase especially at
100 μM apigenin [34]. Taken together, those data suggest that apigenin
possibly modulates cell cycle progression in a dose-dependent and/or cell line
specific manner.
Induction of
autophagy
Autophagy, the
so-called type 2 non-apoptotic cell death, is characterized by the
sequestration of cytoplasmic material into vacuoles for bulk degradation by
lysosomal enzymes. Autophagy is a dynamic process where the cell digests its
own cytoplasmic materials within lysosomes and results in the sequestration and
degradation of macromolecules [35, 36]. There is growing evidence that the relationship
between autophagy and cancer is complex and contradictory. In some cases, autophagy
can serve as a cell survival pathway by providing recycled metabolic substrates
and maintaining energy homeostasis during starvation, while in other settings,
it can cause cell death, either in collaboration with apoptosis or as a backup
mechanism.
Autophagy triggered by apigenin
was first observed in erythroleukemia TF1 cells. Apigenin treatment triggered
the initiation of autophagy without apoptosis [37]. Since then, more evidences have been presented
that apigenin could induce autophagy which serves as tumor suppressive or tumor
protective role under different circumstances [38, 39].
Tong et al. reported that
apigenin exerted its chemopreventive by inducing autophagy in human
keratinocytes via activation of AMPK [40]. In human breast cancer T47D and MDA-MB-231
cells, Cao et al. found that apigenin exposure triggered cell apoptosis and
autophagy as evidenced by the accumulation of acidic vesicular organelles
(AVOs) and LC3-II, a marker of Atg5/Atg7 dependent autophagy. Further, the
authors found that treatment with autophagy inhibitor of 3-MA significantly
enhanced apigenin-triggered apoptosis, suggesting that autophagy induced by
apigenin play a tumor protective role in apigenin-caused cytotoxicity [41]. Similarly, in human colon cancer HCT116 cells,
Lee et al. proved that apigenin concomitantly caused apoptosis and autophagy.
And autophagy played a cell protective role in apigenin-induced cell apoptosis
as well [28].
Beclin-1 regulates the dynamic
autophagic process via the formation of autophagosomes [42, 43]. Beclin-1 is frequently downregulated in many
types of cancers, including solid Ehrlich carcinoma. Gaballah et al. found that
combining 5-FU with apigenin significantly increased Beclin-1 compared with the
vehicle-treated control mice [44]. In addition, Wang et al. showed that apigenin
treatment induced autophagy in macrophages as evidenced by upregulation of
Beclin 1, Atg5, Atg7 and the appearance of LC3-II. And autophagy inhibition by
3-MA pretreatment significantly increased apigenin-induced apoptosis, further
demonstrating that the autophagy triggered by apigenin protected macrophages
from apigenin-induced cytotoxicity [45].
In contrast, in human papillary
thyroid carcinoma BCPAP cells, apigenin exposure resulted in autophagic cell
death associated with p62 degradation and Beclin-1 accumulation and LC3 protein
conversion. Interestingly, co-treatment with 3-MA significantly protected
apigenin-induced cytotoxicity, indicating that apigenin-induced autophagy here
is more likely to be a tumor suppressor [32].
Together, the role
of autophagy in apigenin-induced cytotoxicity depends on cancer cell types. In
most reports, the apigenin-triggered autophagy functions to mediate the
acquired resistance of cancer cells against cell apoptosis, evidenced as
enhanced cell apoptosis induced by apigenin when in cotreatment with autophagy
inhibitors. Under this circumstance, the autophagy plays cytoprotective roles
in apigenin-induced cytotoxicity in cancer cells. In contrast, autophagy acts
as an executioner by inducing autophagic cell death in human papillary thyroid
carcinoma BCPAP cells [32].
Inhibition
of cancer cell migration and invasion
Based on growth
site, tissue origin and growth characteristics, tumors can be divided into
benign tumors and malignant tumors. Benign tumor cells do not have the ability
to migrate. These cells grow and form lesions only in the primary site of the
tumor and can be removed through clinical surgery. However, malignant tumor
cells are highly unstable and have the ability to metastasize and invade other
tissue to form further lesions. The majority of patients in clinical practice
do not die of primary disease but by varying degrees of tumor metastasis.
Presently, metastases, along with the development of chemoresistance and tumor
relapse, are still the major barriers to effective treatment with cancer
therapy. Apigenin has been shown to inhibit cancer cell migration and invasion
in in vitro cancer cells and in vivo animal models.
In prostate cancer DU145 cells,
apigenin strongly inhibited tumor cell invasion and migration in a
dose-dependent manner [46]. With human malignant melanoma cells, 40 µM
apigenin significantly inhibited cell migration and invasion though the
AKT/mTOR pathway in melanoma A375 and C8161 cell lines [14]. In the human lung cancer cell line A549,
apigenin exerted anti-migration and anti-invasion effects by suppressing the
phosphorylation of AKT and targeting the PI3K/AKT signaling pathway [47]. And in the colorectal cancer cell lines DLD1 and
SW480, Dai et al. proved that apigenin could inhibit cell migration, invasion,
and metastasis through modulating the NEDD9/Src/AKT cascade [48].
Moreover, in human
ovarian cancer A2780 cells, apigenin inhibited cancer cell migration and
invasion by decreasing FAK expression in vitro and inhibited spontaneous
metastasis of A2780 cells implanted into the ovary of nude mice in vivo [49]. In addition, in an orthotopic colorectal cancer
model, apigenin prevented cell proliferation and migration by upregulating
transgelin and downregulating MMP-9 expression by decreasing the
phosphorylation of AKT; thus, apigenin inhibited tumor growth and metastasis to
the liver and lung [50].
Induction of
immune responses
Cancer
immunotherapy is a means of treating cancers by activating the patient’s own
immune system. Avoiding destruction by the immune system is a crucial
characteristic for carcinoma cells in overcoming human immune system
surveillance [51]. The programmed cell death 1 (PD1) protein is
commonly expressed in immune cells, such as T cells, B cells, monocytes and
natural killer cells [52, 53]. Its receptors, programmed death ligand 1 (PD-L1)
and 2 (PD-L2), are commonly expressed on the surface of dendritic cells or
macrophages [54, 55]. Recognition and interaction between PD1 and its
ligand will activate PD1 signaling in T cells and blunt the T cell immune
response. Therefore, the PD1/PD-L1 system ensures that the immune system is
activated only at the appropriate time and place and minimizes the possibility
of autoimmune inflammation [56]. PD-L1 is also observed to be highly expressed in
many types of cancer cells and to contribute to cancer cell immune evasion [57]. Therefore, one of the strategies to stimulate
immune surveillance against cancer cells is to target the expression of
PD1/PD-L1 in cancer cells. In human and mouse mammary carcinoma cells, Coombs
et al. proved that apigenin could target STAT1, resulting in the inhibition of
IFN-γ-induced PD-L1 expression. Meanwhile, apigenin treatment induced
PD1-expressing Jurkat T cell proliferation and interleukin-2 synthesis when
co-cultured with MDA-MB-468 cells [58].
Another strategy for cancer
cells to evade immune destruction is to inhibit effector T cells by favoring
the development of T-regulatory cells (Tregs) [59]. In a murine pancreatic cancer model, apigenin
treatment enhanced CD4+CD8+ T cells and decreased the percentage of Tregs,
improving mouse survival time, reducing tumor weights and preventing
splenomegaly. Studies have shown that apigenin potentially stabilized Ikaros
expression in vitro and in vivo by targeting CK2 [60].
Furthermore, apigenin
feeding for 2 weeks resulted in significant suppression of total
immunoglobulin (Ig) E levels, whereas the levels of IgG, IgM and IgA were not
affected in C57BL/6 mice. In addition, apigenin feeding further resulted in the
decreased production of regulated-on-activation normal T cell expressed and
secreted (RANTES) and the soluble tumor necrosis factor receptor I in mouse
serum [61]. In addition, studies have shown that TC-1
tumor-bearing mice that were treated with apigenin combined with E7-HSP70 DNA
were found to generate significant effector and memory E7-specific CD8+ T cell
immune responses, thus generating strong therapeutic anti-tumor effects [62]. Taken together, these findings demonstrate that
apigenin plays a role in cancer immunotherapy. Apigenin shows promise as a
cancer immunotherapy agent by modulating PD1/PD-L1 expression in cancer/T
killer cells and by regulating the percentage of T killer and T regulatory
cells.
Apigenin
functions on cancer stem cells
Apigenin shows
significant cell cytotoxicity selectively against various types of cancer cells
with low or no toxicity to normal cells. These selective anti-cancer effects
are further shown to suppress cancer stem cells (CSCs) in various types of
cancers. CSCs are closely associated with drug resistance, metastasis and the
recurrence of cancer. In the head and neck squamous cell carcinoma cell lines
HN-8, HN-30 and HSC-3, apigenin significantly downregulated the stem cell
markers of CD44, NANOG, and CD105, and abolished the hypoxia-induced increase
in CD44(+) cells, CD105(+) cells and STRO-1(+) cells [63]. In human glioblastoma cells, apigenin inhibited
both the self-renewal capacity, such as cell growth and clonogenicity, and the
invasiveness of GBM stem-like cells by blocking the activation of c-Met signaling
[64]. In addition, in CD44(+) prostate CSCs of PC3
cells, apigenin dose-dependently inhibited cell survival and proliferation by
inducing extrinsic cell apoptosis and increasing cell cycle arrest. Apigenin
also suppressed stem cell migration and adhesion by downregulating matrix
metallopeptidases-2, -9, Snail and Slug. Meanwhile, apigenin treatment
significantly downregulated stemness marker Oct3/4 protein expression by downregulation
of PI3K/Akt and NF-κB signaling pathways. [65]. Sphere-forming cells (SFCs) have self-renewal
capacity and possess stem-like cell properties. Apigenin was demonstrated to
downregulate CK2α expression and inhibited the self-renewal capacity of SFCs in
SKOV3 and HeLa cells [66, 67]. Meanwhile, by targeting CK2, apigenin
synergistically enhanced PI3K/AKT inhibitor-induced apoptosis in CD34(+)CD38(−)
leukemia cells without harming healthy hematopoietic stem cells [68]. Apigenin shows clear anti-cancer effects by
inhibiting the self-renewal capacity of CSCs. This evidence further
demonstrated the effective anti-cancer activities of apigenin. We have noticed
that the current studies of the effects of apigenin on CSCs are mainly
phenomenon descriptions rather than mechanism analyses. However, further
studies of apigenin in cancer therapy are warranted.
Signaling transduction modulation by apigenin
in cancer therapy
Tumorigenesis is
tightly correlated with gene mutation and aberrant cell signaling transduction.
Mutated genes, such as EGFR and Kras, serve as oncogenes, resulting in the
activation of their downstream signaling components and driving the malignant
transformation of normal cells. Therefore, oncogenes and their downstream
signaling pathways are effective targets for cancer therapy. Apigenin has been
reported to target multiple signaling pathways and has been shown as a
promising chemotherapy agent against cancer.
PI3K/AKT/mTOR
signaling pathway
The
phosphatidylinositol 3-kinase (PI3K)/AKT/mammalian target of rapamycin (mTOR)
pathway is one of the most commonly activated signaling pathways, playing a
central role in cell growth, proliferation, migration and differentiation [69–71]. Aberrant activation of these pathways has been
linked to cancer development and is frequently detected in malignancies. Once
activated, AKT phosphorylates a broad range of proteins involved in apoptosis,
cell cycle regulation, growth and survival [72]. Apigenin has been shown to inhibit AKT function
in different cancer cell types by directly suppressing PI3K activity by
blocking the ATP-binding site of PI3K and subsequently inhibiting AKT kinase
activity [73].
Moreover, Ultraviolet B (UVB)
radiation is the major carcinogen for non-melanoma skin cancer by activating
PI3K/AKT/mTOR signaling. Bridgeman et al. demonstrated that apigenin exposure
significantly inhibited UVB-induced mTOR activation and enhanced UVB-induced
autophagy to decrease cell proliferation in mouse keratinocytes. Interestingly,
the mTOR inhibition by apigenin is driven by AMPK activation but not
AKT-dependent [74].
Forkhead box O3 (FOXO3a), a
transcription factor and tumor suppressor, is one of downstream targets of the
PI3K/AKT signaling pathways and is negatively regulated by AKT. Activation of
PI3K/AKT causes FOXO3a phosphorylation, which is related to poor prognosis in a
broad spectrum of cancers [75]. In human breast cancer cells, falvone of
apigenin and luteolin treatment induced FOXO3a expression by suppressing AKT
phosphorylation, and subsequently upregulated the expression of FOXO3a target
genes of p21WAF1/CIP1 and p27KIP1, which resulted in the
inhibition of breast cancer cell proliferation [76]. In addition, apigenin inhibited the human
PI3K/AKT/FOXO signaling pathway in human prostate cancer resulting in cell
cycle arrest and cell apoptosis [77].
Hypoxia is a shortcoming of
radiotherapy in malignant cancers, including laryngeal carcinoma. GLUT-1 is an
important marker in hypoxia-induced therapies. Apigenin has the potential to
decrease GLUT-1 expression levels via downregulation of the PI3K/AKT signaling
pathway in vitro and in vivo, which enhances xenograft radiosensitivity and
inhibits tumor growth [78].
Interestingly,
apigenin can activate PI3K/AKT/mTOR signaling pathway and protect
cardiomyocytes from chemotherapy-caused cytotoxicity in mice. Adriamycin is
widely used in clinic for treatment of various types of cancers. However, the
severe cardiotoxic side effects caused by adriamycin limited its usage in
cancer therapy. Yu et al. reported that apigenin alleviated adriamycin-induced
cardiotoxic by activating PI3K/AKT/mTOR pathway which inhibited
adriamycin-induced cardiomyocyte apoptosis and autophagy [79]. Those data further demonstrate that apigenin has
selective anti-cancer efficacy and low or no side effects to normal cells.
MAPK/ERK signaling
The
mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase
(ERK) signaling pathway is another oncogenic pathway that is frequently
hyperactivated in cancer, deregulating the control of metabolism, cell
apoptosis, survival and proliferation [80]. The mutation or overexpression of receptor
tyrosine kinases and Ras will inevitably cause hyperactivation of this pathway.
Therefore, the components of this signaling pathway are ideal therapeutic
targets for cancer therapy. In addition to inhibiting the PI3K/AKT signaling
pathway, apigenin was proven to modulate MAPK/ERK signaling pathway in various
cancers in vitro and in vivo. In human melanoma A375 and C8161 cell lines,
apigenin effectively suppressed cell proliferation, migration and invasion and
induced G2/M phase arrest and apoptosis via decreasing p-ERK1/2, p-AKT and
p-mTOR [14]. In non-small cell lung cancer cells, apigenin
enhanced TRAIL-induced apoptosis by modulating DR4/DR5, AKT, ERK and NF-κB
signaling [26]. And in the colorectal cancer cell lines HCT116
and DLD1, apigenin was found to enhance ABT-263-induced antitumor activity via
the inhibition of the prosurvival regulators AKT and ERK in vitro and in vivo [24]. In addition, the co-inhibition of AKT and ERK
signaling was observed in an autochthonous mouse prostate cancer model.
Apigenin administration effectively suppressed prostate cancer progression in
those mice by decreasing IGF/IGFBP-3 and inhibiting p-AKT and p-ERK1/2 [81].
In other studies, ERK signaling
pathway was inhibited by apigenin along with other protein kinases, such as
focal adhesion kinase (FAK). Hasnat et al. showed that apigenin induced anoikis
in human cutaneous melanoma cells by reducing integrin protein levels and
inhibiting the phosphorylation of both FAK and ERK1/2 [82]. In addition, in pancreatic cancer cells,
apigenin targeted FAK and ERK to suppress the effects of
4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone on pancreatic cancer cell
proliferation and migration [83].
Meanwhile, ERK was
not inhibited but activated by apigenin in few studies. In human prostate
cancer LNCaP and PC-3 cells, Shukla et al. demonstrated that apigenin treatment
increased phosphorylation of ERK1/2 and JNK1/2 and decreased phosphorylation of
p38. The modulation of MAPKs by apigenin contributed to apigenin-induced cell
cycle arrest at G0/G1 phase [84]. And in human choriocarcinoma cells, apigenin was
found to induce cell apoptosis and reduce cell survival by suppressing the
AKT-mTOR pathway and increasing the phosphorylation of ERK1/2 and P90RSK in a
dose-dependent manner [85].
NF-κB
signaling pathway
The Nuclear
Factor-κappa B (NF-κB) signaling pathway is generally considered an active
factor in survival and proliferation. There are several homodimer and
heterodimer forms of NF-κB: NF-κB1 (p50/p105), NF-κB2 (p52/100), RelA (p65),
RelB, and the c-Rel proteins [86]. In addition, the NF-κB family members are
regulated by the IκB protein members (IκBα, IκBβ, IκBε, IkBγ, Bcl-3, p100, and
p105). NF-κB is a heterodimer in the cytoplasm and binds to IκB in an inactive
state. Many different signal molecules, such as TNF, FasL and TRAIL, cause IKK
complex activation, resulting in IκBα phosphorylation and degradation by the
proteasome. Then, NF-κB is released and translocated to the nucleus. NF-κB
inhibits cell death via the activation of target genes, which include
prosurvival genes (Bcl-2, Bcl-xL, survivin, XIAP), cell cycle-related genes
(cyclin D1), VEGF, inflammatory cytokines, and tumor metastasis genes (COX-2) [87, 88].
In most cases, apigenin
treatment inhibits NF-κB activation both in vitro and in vivo. In a prostate
TRAMP mouse model, Shukla et al. showed that apigenin feeding to TRAMP mice
inhibited prostate tumorigenesis by interfering with NF-κB signaling. Apigenin
administration significantly decreased prostate tumor volumes and completely abolished
cancer cell metastasis. Studies have shown that apigenin administration blocked
the phosphorylation and degradation of IκBα by inhibiting IKK activation, which
in turn led to the suppression of NF-κB activation [89]. Further, Shukla and colleagues proved that
apigenin was a specific IKK inhibitor by directly binding with IKKα to
attenuate its kinase activity, thereby suppressing NF-κB/p65 activation in the
human prostate cancer cell lines PC-3 and 22Rv1. In addition, the inhibitory
efficacy of apigenin on IKKα is much more effective than PS1145, a specific IKK
inhibitor [90].
Moreover, in the human
non-small cell lung cancer cell line A549, apigenin did not affect the
expression of NF-κB but suppressed the translocation of NF-κB from the
cytoplasm to the nucleus, which further inhibited target genes, such as Bcl-2,
Mcl-1 and Bcl-xL, that block apoptosis. Apigenin also blocks the degradation of
IκBα in lung cancers, which further blocks the separation of IκBα from the
NF-κB heterodimer [26]. And in malignant mesothelioma, apigenin
treatment showed anti-cancer effects in vitro and in vivo by inhibiting NF-κB
nuclear translocation and AKT activation and modulating MAPK signaling pathways
[91].
JAK/STAT
signaling
The signal
transducer and activator of transcription (STAT) proteins are members of a
family of transcription factors that mediate signals from cytokines and growth
factors to regulate cell proliferation and differentiation. STAT activation is
usually mediated by non-receptor tyrosine kinase members of the Janus kinase
(JAK) family [92]. JAK/STAT signaling is constantly activated in
various human carcinomas and promotes tumorigenesis and metastasis by promoting
the expression of genes that encode antiapoptotic proteins, cell cycle
regulators, and angiogenic factors [93, 94]. Moreover, STAT3 can also be activated by
tyrosine kinase receptors, such as EGFR and c-MET [95, 96]. Therefore, targeting STAT members is considered
and evaluated as a promising therapeutic strategy in various cancer therapies.
In the murine melanoma B16F10
cell line, apigenin showed anti-metastatic activity via STAT3 phosphorylation
inhibition. Meanwhile, the authors found that apigenin downregulated the STAT3
target genes MMP-2, MMP-9, VEGF and Twist1, which are important for cell
migration and invasion [97]. In human myeloid leukemia HL60 cells and
erythroid leukemia TF1 cells, the JAK/STAT pathway was one of the major targets
of apigenin. Apigenin decreased the phosphorylation of JAK2 and STAT3 in HL60
and TF1 cells and decreased STAT5 in TF1 cells. The decrease in JAK/STAT
phosphorylation enhanced apigenin-induced leukemia cytotoxicity [37]. Additionally, in the human HER2-overexpressing
breast cancer cell lines BT-474, SKBR3 and MDA-MB-453, Seo et al. found that
apigenin triggered cell apoptosis by suppressing JAK/STAT3 signaling and
decreasing nuclear translocation of STAT3 [98, 99]. Therefore, apigenin is one of the agents that
can effectively target STAT signaling pathways.
Besides, JAK/STAT
signaling pathway was not affected by apigenin in some reports. In human
ovarian cancer SKOV3 cells and the chemoresistant ovarian cancer SKOV3/TR
cells, apigenin significantly decreased Axl and Tyro3 receptor tyrosine kinase
at both RNA and protein levels, without changing the IL-6 production and
phospho-STAT3 protein levels [100].
Wnt/β-catenin
signaling
Wnt/β-catenin
signaling is highly conserved from sponge to human and plays important roles in
metazoan development and tissue homeostasis. Dysregulation of this signaling
pathway leads to the accumulation of β-catenin in the nucleus and is linked to
several human diseases, including cancer [101, 102]. The increased expression of Wnt, frizzle or
lymphoid enhancer factor (LEF)/T cell factor (TCF) in this signaling pathway is
commonly detected in patients with leukemia, colorectal cancer, breast cancer
or adrenocortical tumors [103–106]. In addition, targeting Wnt/β-catenin signaling
by shRNA or the overexpression of domain-negative β-catenin or TCF has been
found to suppress tumor cell growth and has become a new strategy for cancer
treatment [107]. Apigenin was found to significantly inhibit the
Wnt/β-catenin signaling pathway, thereby suppressing cell proliferation,
migration, and invasion in colorectal and osteosarcoma cancers [11, 108]. Recently, Lin et al. reported that apigenin
downregulated total, cytoplasmic and nuclear β-catenin through the induction of
the autophagy-lysosomal system. Furthermore, they proved that the autolysosomal
degradation of β-catenin by apigenin occurred via inhibition of the AKT/mTOR
signaling pathway. In addition, treatment with the autophagy inhibitors
wortmannin and chloroquine restored the accumulation of β-catenin in the cell
nucleus, indicating the involvement of the autophagy-lysosomal system in the
degradation of β-catenin [109].
In addition to the
signaling pathways mentioned above, there is evidence of apigenin involvement
in other signaling pathways. Those signaling pathways include AMPK [40], transforming growth factor-β (TGF-β) [110, 111], JNK [44, 112] and FAK [82, 83]. All of these proteins and signaling pathways are
potential therapeutic targets for cancer treatment. Apigenin functions as a
promising chemotherapy agent that is able to effectively target multiple
signaling pathways. And the modulation of these signaling pathways by apigenin
induces cancer cell apoptosis or autophagy and attenuates cancer cell
proliferation or metastasis.
Combination therapy for apigenin
Given that cancer
cells have multiple genetic alterations, a combinatorial therapeutic strategy
is demanded for effective cancer therapy. The main purposes of the
combinatorial strategy for cancer therapy are to potentiate the antitumor
effects of chemotherapeutic agents and to overcome the limitation of acquired
drug resistance. Apigenin is an effective anti-cancer agent but with only
moderate anti-cancer efficacy when used alone at human physiological dosages [24, 38, 113]. Therefore, co-treatment with other chemodrugs is
a reasonable way to enhance its anti-cancer activities. The combination of
apigenin and other chemodrugs are summarized in Table 2. In addition, most of the combination
treatments resulted in enhanced anti-cancer efficacy in vitro and in vivo.
Table 2
The combination therapy by
apigenin and other chemodrugs
Cotreatment partner
|
Tumor type
|
Cell lines (concentration)
|
Combination effects
|
Mechanisms
|
Citations
|
IFNγ
|
Cervical cancer
|
HeLa and SiHa (10 μM)
|
Enhance the anticancer activity
|
Targeting cyclin-dependent kinase 1
|
[114]
|
Paclitaxel
|
Ovarian cancer
|
SKOV3 (40 μM)
|
Overcome taxol resistance
|
Downregulation of Axl and Tyro3 RTKs expression
|
[100]
|
Cisplatin
|
Multiple tumor types
|
HeLa, A549, HCT 116, H1299, and MCF-7 (30 µM)
|
Enhances the cisplatin cytotoxic effect
|
Increased DNA damage in a p53-dependent manner
|
[116]
|
Prostate cancer stem cells
|
PC3 and CSCs (15 μM)
|
Enhance anticancer effects
|
Suppressed PI3K/AKT activation and protein
expression of NF-κB
|
[117]
|
|
Laryngeal carcinoma
|
Hep-2 (40 μM)
|
Enhance the sensitivity to cisplatin
|
Inhibition of GLUT-1 and p-AKT
|
[119]
|
|
Solid Ehrlich carcinoma
|
Swiss male albino mice, intraperitoneally
(100 mg/kg)
|
Enhanced anti-cancer effect
|
Increased Beclin-1, caspases 3, 9 and JNK activities
and decreased Mcl-1
|
[44]
|
|
5-Fluorouracil (5-FU)
|
Hepatocellular carcinoma
|
SK-Hep-1 and BEL-7402 (4 μM)
|
Enhanced anticancer activity
|
Inhibition of ROS-mediated drug resistance and
decreased Bcl-2 expression and loss of ΔΨm
|
[120]
|
Pancreatic cancer
|
BxPC-3 (13 μM)
|
Potentiate anti-proliferative effect
|
Decreased nuclear GSK-3β and NF-κB p65
|
[121]
|
|
Doxorubicin and etoposide
|
Leukaemia
|
CCRF-CEM and Jurkat (10 μM)
|
Enhancing cell cytotoxicity
|
Increased DNA damage
|
[122]
|
TRAIL
|
Non-small cell lung cancer
|
A549 and H1299 (20 μM)
|
Enhance anti-tumor activity
|
Upregulated DR4/DR5 expression in a p53-dependent
manner
|
[26]
|
Anaplastic thyroid carcinoma
|
8505C and CAL62 (40 μM)
|
Potentiates synergistic cytotoxicity
|
Reduced Bcl-2 and inactivation of ERK
|
[125]
|
|
Prostate cancer
|
DU145 (20 μM)
|
Enhancing cell apoptosis
|
Targeting adenine nucleotide translocase-2
|
[124]
|
|
ABT-263
|
Colon cancer
|
HCT116 and DLD1 (20 µM)
|
Enhance cell apoptosis
|
Inhibition of AKT and ERK signaling and Mcl-1 and
upregulation of Bim
|
[24]
|
miR-433-5p knockdown
|
Glioma stem cell
|
CD133-positive GSCs (20 μM)
|
Enhance cell apoptosis
|
Changes in Bax/Bcl-2 ratio, increased cytochrome c
level, Apaf-1 induction, and caspase-3 activation
|
[126]
|
miR-138
|
Neuroblastoma
|
SK-N-DZ and SK-N-BE2 (100 μM)
|
Enhance cell apoptosis inhibition of cell viability
|
Increased Bax/Bcl-2 ratio and caspase-3,8
|
[127]
|
4-Hydroxy-2-nonenal (4-HNE)
|
The rat adrenal pheochromocytoma
|
PC12 (20 μM)
|
Attenuate 4-HNE-mediated cell death
|
Restore 4-HNE-induced ER homeostasis through
modulating of UPR, Nrf2-ARE and MAPK pathways
|
[128]
|
Chemotherapy drugs, such as
cisplatin and paclitaxel, are widely used in the clinic for cancer control.
These drugs play a considerable role in the extension of the overall survival
rates of cancer patients; however, their undesired toxicity has always been a
matter of concern for clinicians and patients. To enhance their antitumor
effects and to minimize their limitation, co-administration with other targeted
drugs has been widely tested and has achieved great success in clinical
applications. Studies have shown that co-administration with apigenin
significantly enhances the anti-cancer efficacy of chemodrugs and helps
overcome their limitations in various types of cancers by targeting multiple
signaling pathways (Table 2) [44, 114–122].
Recombinant Apo2L/tumor
necrosis factor-related apoptosis-inducing ligand (TRAIL) is an effective
antitumor agent that induces cancer cell death without damaging normal cells
and that has been evaluated in clinical trials. However, TRAIL treatment only
showed limited anti-cancer activity in many malignant cancers because of
acquired resistance [123]. Overcoming this resistance is essential for
chemotherapy using the Apo2L/TRAIL pathway. In prostate cancer DU145 and LNCaP
cells, apigenin overcomes resistance to Apo2L/TRAIL by inhibiting adenine
nucleotide translocase-2 (ANT2) and upregulating DR5. Further, silencing of
ANT2 by siRNA lowered the enhancement of DR5 expression by apigenin, indicating
that ANT2 inhibition is needed for apigenin to enhance DR5 expression and
Apo2L/TRAIL-induced apoptosis [26, 124]. NSCLC A549 and H1299 cells are resistant to TRAIL
treatment alone. Apigenin exposure upregulates DR4 and DR5 expression and
sensitizes those cells to TRAIL-induced apoptosis in a p53-dependent manner [26, 124]. Furthermore, Kim et al. showed that apigenin
synergistically enhanced the cytotoxicity of TRAIL in anaplastic thyroid
carcinoma (ATC) cells by modulating the Bcl-2 family proteins [125].
MicroRNAs (miRNAs) are short
non-coding RNAs of 20–24 nucleotides that function in post-transcriptional
regulation of gene expression. Aberrant miRNA expression may affect a multitude
of transcripts and profoundly influence cancer-related signaling pathways.
Therefore, miRNAs may function as tumor suppressors or oncogenes involved in
the pathogenesis of tumors. Modulation of miRNA expression could also enhance
apigenin-induced antitumor effects. miR-423-5p is overexpressed in
glioblastomas and contributes to glioma stem cells. The downregulation of
miR-423-5p enhances apigenin-induced cell apoptosis in glioma stem cells with a
shift in the Bax/Bcl-2 ratio, an increased cytochrome c level, Apaf-1 induction
and caspase-3 activation [126]. In contrast, in malignant neuroblastoma SK-N-DZ
and SK-N-BE2 cells, miR-138 overexpression significantly enhanced apigenin-induced
cell apoptosis and decreased cell viability and colony formation capability in
vitro and effectively suppressed tumor growth in vivo [127].
Alleviating the side
effects of one drug by co-treatment with a second agent is also a widely used
strategy in cancer therapy. It is known that apigenin exhibits a broad spectrum
of biological activities, including antioxidant and anti-inflammatory
activities. Nephrotoxicity is one of the adverse effects that limits the usage
of cisplatin in cancer therapy. Hassan et al. found that co-administration with
apigenin significantly reduced blood urea nitrogen, serum creatinine, TNF-α,
IL-6, COXI, COXII, and MDA levels and increased GSH levels, thereby protecting
Wistar Albino mice from cisplatin-induced nephrotoxicity [118]. One lipid peroxidation product that is
implicated as a causative factor to cause neurodegenerative disorders is
4-hydroxy-2-nonenal (4-HNE). In another study, apigenin significantly
attenuated 4-HNE-mediated cell death in neuronal-like catecholaminergic PC12
cells via restoration of ER homeostasis [128]. Therefore, apigenin can not only be used as an
adjuvant chemotherapeutic agent to overcome drug resistant but also show
significant protective effects and alleviate chemodrug-mediated adverse
effects.
Conclusions
As a naturally
occurring flavonoid compound, apigenin not only has low toxicity
characteristics but also plays an important role in a variety of ways. All
evidence gathered thus far clearly indicates that apigenin has strong
anti-cancer activities against various human cancers alone and in combination
with other chemotherapeutic agents. It is worthy to note that in most cases
apigenin treatment can concomitantly cause multiple anti-cancer effects in the
same treatment. For example, In ACC cells, apigenin suppressed ACC-2 cell
survival by inducing both apoptosis and G2/M-phase arrest in a dose- and
time-dependent manner [28]. And in the human colon cancer HCT116 cells,
apigenin treatment triggered both autophagy and apoptosis [28]. Furthermore, in human melanoma cells, Zhao et
al. reported that apigenin showed effective antitumor effects as suppression of
cell migration and invasion, induction of cell cycle arrest at G2/M phase and
triggering cell apoptosis simultaneously [11]. These different antitumor effects simultaneously
triggered by apigenin demonstrated that apigenin has a wide range of antitumor
effects, but also the results that apigenin can simultaneously target a variety
of signal pathways and protein kinase. As summarized in Fig. 2, the same
signal pathway inhibition will also lead to different antitumor effects.
Apigenin shows antitumor
activities by modulating multiple signaling pathways, including PI3K/AKT,
NF-kB, JAK/STATs, Wnt/β-catenin, AMPK, MAPK/ERK, and JNK. We need to note that
although so many signaling pathways are reported to be modulated by apigenin, it
is still not clear whether there is cross regulation among those signaling
pathways. In addition, it is unclear how apigenin functions to modulate those
signaling pathways. To determine the direct targets of apigenin, Arango et al.
carried out high-throughput screening of phage display coupled with second
generation sequencing and identified a group of 160 potential targets of
apigenin. Those targeted proteins are significantly enriched in three main
functional categories: GTPase activation, membrane transport, and mRNA
metabolism/alternative splicing [129]. However, whether the signaling pathways
modulated by apigenin are regulated through those direct targets still needs further
exploration.
Though apigenin was widely
investigated with xenograft models in mice for its anticancer effects, few
reports mentioned that apigenin caused side effects to animals. To better
develop and utilize apigenin in cancer therapy, the potential toxicity of
apigenin was investigated in Swiss mice with acute exposure test [130]. Apigenin was administered intraperitoneally at
doses of 25, 50, 100 and 200 mg/kg. Twenty-four hours later, mice were
sacrificed and blood and liver tissues were collected for further analysis.
Singh et al. found that doses of 100 or 200 mg/kg but not 25 or
50 mg/kg apigenin showed liver toxicity, evidenced as increased ALT, AST,
ALP in serum and increased ROS, ratio of oxidized to reduced glutathione
(GSSG/GSH) and LPO, and altered enzyme activities along with damaged
histoarchitecture in the liver tissue [130]. This warrants the doses of apigenin by
intraperitoneal route in vivo.
Until now, the
anticancer effects of apigenin have been mainly studied in in vitro cancer
cells and preclinical animal models. There are still no clinical data on
apigenin in human cancer therapy. The good news is that the pharmacological
effects of apigenin as a dietary supplement are under evaluation in a phase 2
clinical study by Technische Universität Dresden. The bioflavonoid mixture,
with 20 mg apigenin and 20 mg epigallocatechin gallate, is served as
a daily nutritional supplement to patients with resected colorectal carcinomas
to evaluate the prevention of the recurrence of neoplasia [https://clinicaltrials.gov/ct2/show/NCT00609310?term=Technische±Universit%C3%A4t±Dresden±apigenin&rank=1].
With more evidence, an in-depth understanding of apigenin in cancer therapy and
more in-depth studies on its pharmacological mechanism and toxicology in
different cancers, further clinical studies on apigenin in cancer therapy will
be warranted. Apigenin appears to have the potential to be developed either as
a dietary supplement or as an adjuvant chemotherapeutic agent for cancer
therapy.
Abbreviations
NSCLC
|
non-small
cell lung cancer
|
DR4
|
death
receptor 4
|
DR5
|
death receptor 5
|
CDK1
|
cyclin-telangiectasia
mutated
|
GLUT-1
|
glucose transporter-1
|
ROS
|
reactive
oxygen species
|
AVOs
|
acidic vesicular organelles
|
PD1
|
programmed
cell death 1
|
PD-L1
|
programmed death ligand 1; P
dependent kinase-1
|
PCNA
|
proliferating
cell nuclear antigen
|
ACC
|
adenoid cystic carcinoma
|
ATM
|
ataxia
|
D-L2
|
programmed death ligand 2
|
Tregs
|
T-regulatory
cells
|
Ig
|
immunoglobulin
|
RANTES
|
regulated-on-activation
normal T cell expressed and secreted
|
PI3K
|
phosphatidylinositol 3-kinase
|
mTOR
|
mammalian
target of rapamycin
|
UVB
|
ultraviolet B
|
FOXO3a
|
forkhead box
O3
|
MAPK
|
mitogen-activated protein kinase
|
ERK
|
extracellular
signal-regulated kinase
|
FAK
|
focal adhesion kinase
|
NF-κB
|
nuclear
factor-κappa B
|
STAT
|
signal transducer and activator of
transcription
|
JAK
|
Janus kinase
|
LEF
|
lymphoid enhancer factor
|
TCF
|
T cell factor
|
TGF-β
|
transforming growth factor-β
|
TRAIL
|
tumor
necrosis factor-related apoptosis-inducing ligand
|
ANT2
|
adenine nucleotide translocase-2
|
ATC
|
anaplastic
thyroid carcinoma
|
miRNAs
|
microRNAs
|
4-HNE
|
4-hydroxy-2-nonenal
|
CSCs
|
cancer stem cells
|
SFCs
|
sphere-forming
cells
|