Key words
general methods - cancer
Abbreviations
ABC transporter:
ATP-binding cassette transporter
BFU-E:
erythroid burst forming units
BJBDT:
Bo-jung-bang-dock-tang
CD:
cluster of differentiation
CFU-GEMM:
granulocyte erythrocyte monocyte macrophage colony-forming units
CFU-S:
spleen colony forming units
CPA:
cyclophosphamide
EPO:
erythropoietin
ESC:
embryonic stem cells
FAS:
fatty acid synthase
GAS:
?-interferon activated sites
GM-CSF:
granulocyte-macrophage-colony stimulating factor
HCC:
hepatocellular carcinoma
hMSC:
human bone marrow mesenchymal stem cells
HSC:
hematopoietic stem cells
IL:
interleukin
iPSC:
induced pluripotent stem cells
JAK:
Janus activated kinase
KMKKT:
Ka-mi-kae-kyuk-tang
SCF:
stem cell factor
STAT:
signal transducer and activator of transcription
T-IC:
tumor-initiating cells
TPO:
thrombopoietin
Embryonic Stem Cells and Pluripotent Adult Stem Cells for Tissue Regeneration
Embryonic Stem Cells and Pluripotent Adult Stem Cells for Tissue Regeneration
Somatic stem cells can be found in many rapidly regenerating tissues, e.g., the skin,
gastrointestinal mucosa, and hematopoietic system, but are also present at low numbers
in non-regenerative organs such as the heart and brain. They function in maintaining
normal tissue homeostasis and in tissue repair after injury as well as self-renewal
and the generation of specific progenitor cells during differentiation. If needed,
somatic stem cells can switch from a quiescent state to a highly proliferative state,
differentiate to functionally mature cells and replace other differentiated cells
during physiological tissue turnover [1], [2], [3], [4].
One of the most promising applications of stem cell biology is tissue regeneration.
The concept is to differentiate stem cells into specific tissue and cell types in
order to treat neurodegenerative diseases, muscular diseases, diabetes, and others.
The challenge in regenerative medicine is the targeted differentiation of stem cells
into the desired cell type. Embryonic stem cell-like induced pluripotent stem cells
(iPSC) can be generated by inducing expression of defined transcription factors. The
expansion and differentiation of stem cells takes place in vitro in cell cultures using cocktails of growth factors and natural or synthetic small
molecules, which regulate crucial signaling pathways of stem cell differentiation
[5]. Current protocols are based on embryonic stem cells (mainly derived from aborted
fetuses) or adult stem cells (i.e., hematopoietic stem cells derived from bone marrow)
[6].
Cancer Stem-like Cells and Their Role in Cancer Therapy
Cancer Stem-like Cells and Their Role in Cancer Therapy
Cancer stem cells are small populations of self-renewing cells with the ability to
proliferate upon appropriate stimulation and differentiate to heterogeneous lineages
in tumors [7]. The term “cancer stem cells” is merely an operational term. Cancer stem cells do
not necessarily derive from normal stem cells nor are they multipotent [8], [9]. Hence, cancer stem cells may not show lineage-dependent cell differentiation. Therefore,
terms such as “cancer-initiating cells” or “cancer stem-like cells” are more appropriate
than “cancer stem cells”.
Cancer stem-like cells are characterized by clonal expansion that initiates and sustains
tumor growth in vivo. Cancer stem-like cells should not be confused with the cell of cancer origin, which
is the very first malignantly transformed cell [10]. Cancer stem-like cells can originate from normal stem cells or progenitor cells
after acquisition of tumor-initiating mutations [11], [12]. It remains a matter of discussion whether cancer stem-like cells can be derived
only from normal stem cells [13], [14], or if they can also be derived from mature cells that are dedifferentiated or transdifferentiated,
which is regulated by genetic and epigenetic factors [9], [15].
There is much debate about the identification of cancer stem-like cells, and a variety
of different cellular markers have been proposed to differentiate between cancer stem-like
cells and the general population of cancer cells within a tumor. Putative markers
of high relevance include the cell surface protein CD34 and the glycosylated isoform
of CD133/prominin [16]. Another marker, e.g., in breast cancer, is the hyaluronic acid binding receptor
CD44 which controls cell-cell interactions and is involved in cross-talk between cancer
stem-like cells and their microenvironment [17], [18], [19]. CD44+/CD24- cells are associated with breast cancer metastasis to distant sites
[20]. CD24 inhibits CXCL12-CXCR4-mediated migration of cells. Loss of CD24 expression,
therefore, fosters invasion and metastasis [21]. However, these markers are not exclusively expressed by cancer stem-like cells,
but rather are also expressed on normal cells. Furthermore, cancer stem-like cells
have been identified that lack these markers [22].
The development of resistance represents a major obstacle to successful cancer therapy.
Current protocols in radio- and chemotherapy are able to remove bulk cancer tissue.
However, therapy-resistant cancer stem-like cells may survive and resume growth at
a later time, leading to disease recurrence and metastasis. This phenomenon, termed
minimal residual disease, has been known for many decades. Cancer stem-like cells
may be able to survive radio- and chemotherapy by adopting a non-proliferative, dormant
state [23], [24]. Even after years, cancer stem-like cells can be reactivated and lead to refractory
disease [25], [26].
Cancer stem-like cells often reside in niches within tissues where they are shielded
against the stresses of chemotherapy [27], [28], [29]. Hypoxic areas serve as niches, and hypoxic tumors are known to be radio- and drug-resistant
[30]. Furthermore, hypoxic areas are less vascularized than oxygenated ones. Therefore,
antiangiogenic treatment approaches may affect the niches of cancer stem-like cells
[31].
Several mechanisms of drug resistance have been found in cancer stem-like cells:
-
High expression levels of drug transporters of the ATP-binding cassette (ABC) type,
particularly ABCB1/MDR1/P-gp and ABCG2/BCRP, especially in side populations [32], [33], [34]
-
Highly efficient DNA repair [35]
-
Resistance to commit programmed cell death (apoptosis) [36]
-
Quiescence. Non-proliferating cells are generally more resistant than proliferating
ones.
-
Decreased immunogenicity [36].
Activity of Natural Products On Embryonic Stem Cells
Activity of Natural Products On Embryonic Stem Cells
Several laboratories have screened small molecule libraries for compounds that promote
embryonic stem cell differentiation. Embryonic stem cells (ESCs) are an attractive
source of cells for disease modeling in vitro and may eventually provide access to cells/tissues for the treatment of degenerative
diseases. Stauprimide, a staurosporine derivative, primes ESCs for efficient differentiation
through a mechanism that affects expression of the transcription factor and oncogene,
c-Myc [37]. Staurosporine is a known kinase inhibitor [38].
Retinoic acid acts by binding to nuclear receptors and inducing transcription of specific
target genes during embryonic stem cell and embryonic carcinoma differentiation. The
standard murine EC cell line F9 can be induced by retinoic acid to differentiate into
primitive, parietal, and visceral endodermal cells. Another EC cell line, P19, differentiated
into endodermal and neuronal cells upon retinoic acid treatment [39].
Butyrate, a natural small fatty acid and histone deacetylase inhibitor, significantly
increased the efficiency of mouse iPSC generation using the transcription factors
Oct4, Sox2, Klf4, and c-Myc. Butyrate not only changed the reprogramming dynamics
by reducing the frequency of partial reprogramming and transformation, but also increased
the ratio of iPSC colonies to total colonies. This effect was mediated by c-Myc during
early stages of reprogramming [40].
Epidemiological evidence indicates that fruits and vegetables possess chemopreventive
activity against cancer. Therefore, it is possible that natural products may selectively
target cancer stem cells and induce differentiation. Reynertson et al. [41] used an alkaline phosphatase stain to assay plant extracts for the ability to induce
differentiation in embryonic stem cells. Alkaline phosphatase is a characteristic
marker of undifferentiated embryonic stem cells. The authors investigated approximately
100 fractions obtained from 12 species of ethnopharmacologically used plants and found
fractions from 3 species that induced differentiation, decreasing the levels of alkaline
phosphatase and pluripotency marker transcripts (Nanog, Oct-4, Rex-1). These fractions
affected proliferation of murine embryonic stem cells, human embryonic cells, and
prostate and breast carcinoma cells in a concentration-dependent manner. The isolated
ellagic acid and gallic acid were cytotoxic towards cultured breast carcinoma cells.
Effects of Natural Products On Pluripotent Adult Stem Cells
Effects of Natural Products On Pluripotent Adult Stem Cells
Reprogramming of pluripotent adult stem cells
Vitamin C enhanced iPSC generation from both mouse and human somatic cells at least
in part by alleviating cell senescence, an important roadblock to reprogramming. In
addition, vitamin C accelerated gene expression changes and promoted the transition
of pre-iPSC colonies to a fully reprogrammed state [42].
Hematopoietic stem cells in high-dose chemotherapy
Hematopoietic stem cell transplantation is employed in oncology mainly for treatment
of lymphoproliferative disorders and leukemia. Multipotent hematopoietic stem cells
derived from bone marrow, peripheral blood stem cells, or umbilical cord blood are
preserved before the patient is subjected to high doses of ablative chemotherapy in
which tumor cells and normal proliferating hematopoietic cells are destroyed. Reinfusion
of hematopoietic stem cells induces repopulation of the bone marrow and reconstitution
of the blood.
The Kampo (Japanese herbal) extract TJ-48 accelerated recovery from hematopoietic
injury induced by radiation and the anti-cancer drug mitomycin C. The active fraction
of TJ-48 is composed of free fatty acids (oleic acid and linolenic acid). Among a
panel of 27 different free fatty acids, oleic acid, elaidic acid, and linolenic acid
showed potent activity on hematopoietic stem cells. The administration of oleic acid
to mitomycin C-treated mice enhanced spleen colony forming units (CFU-S) counts to
twice those of the control group when counted on days 8 and 14, suggesting that the
fatty acids contained in TJ-48 actively promote the proliferation of hematopoietic
stem cells [43].
Bo-jung-bang-dock-tang (BJBDT) is a medicinal herbal cocktail that has been used for
cancer prevention and treatment in traditional Korean medicine. Lim et al. [44] suggested that BJBDT can enhance hematopoiesis via the hematopoietic cytokine-mediated
JAK2/STAT5 pathway. BJBDT significantly increased expression of the hematopoietic
cytokines interleukin (IL-3), stem cell factor (SCF), granulocyte-macrophage-colony
stimulating factor (GM-CSF), thrombopoietin (TPO), and erythropoietin (EPO) at the
level of mRNA and secretion in hematopoietic stem cells (HSCs). Additionally, BJBDT
enhanced the phosphorylation of Janus activated kinase 2 (JAK2) and the signal transducer
and activator of transcription 5 (STAT5) as well as STAT binding to γ-interferon activated sites (GAS) in HSCs. Furthermore, BJBDT significantly enhanced
the growth rate of granulocyte erythrocyte monocyte macrophage colony-forming units
(CFU-GEMM) and erythroid burst forming units (BFU-E) in vitro. Moreover, BJBDT increased EPO at the mRNA level in the kidney and plasma. It also
increased the number of erythroid-specific antigen Ter-119(+) erythroid cells in mice
with aplastic anemia induced by 20 % benzene. Consistently, histochemical staining
revealed that BJBDT increased the bone marrow and stromal cell populations and decreased
macrophage and adipocyte populations in bone marrow tissue of mice with aplastic anemia.
Ka-mi-kae-kyuk-tang (KMKKT) is an Oriental herbal medicinal cocktail that has demonstrated
antiangiogenic, anticancer, and antimetastatic activities without considerable side
effects in preclinical animal models. Seo et al. [45] investigated whether KMKKT alleviated cancer chemotherapy-induced leukopenia or
other hematotoxicities in vivo using a mouse model. KMKKT was given orally once daily for 10 days to the mice before
they were given daily injection of cyclophosphamide (CPA) for 4 days. KMKKT ameliorated
the CPA-induced decrease in red blood cells, hemoglobin content, and total white blood
cell/leukocyte counts. Examination of multiple organ sites involved in hematopoiesis
and lymphocyte differentiation and maturation revealed that the changes induced by
CPA had been attenuated in each and every type of cell examined. Some cell types in
the bone marrow were fully restored. In particular, bone marrow stem cells in the
Sca-1(+), CD117(+), Sca1(+)/CD117(+), and CD34(+)/CD117(+) mice were overstimulated,
supporting a role for KMKKT in stimulating hematopoietic stem cell (HSC) signaling
to compensate for CPA-induced destruction of leukocytes and other cell types.
Pluripotent stem cells of normal tissue in cancer therapy
Multiple myeloma is characterized by the accumulation of clonal malignant plasma cells
in the bone marrow, which stimulates bone destruction by osteoclasts and reduces bone
formation by osteoblasts. In turn, the altered bone microenvironment sustains survival
of myeloma cells. A major challenge in treating multiple myeloma is discovering drugs
that target not only myeloma cells but also osteoclasts and osteoblasts. Resveratrol
promotes the expression of osteoblast markers like osteocalcin and osteopontin in
human bone marrow mesenchymal stem cells (hMSC-TERT) in a concentration-dependent
manner and stimulates their response to 1,25(OH)2 vitamin D3 [1,25(OH)2D3] [46]. Moreover, resveratrol upregulates the expression of 1,25(OH)2D3 nuclear receptor
in a concentration-dependent manner.
PHY906 is a formulation of four herbal compounds traditionally used to treat nausea,
vomiting, cramping, and diarrhea. Irinotecan is an established anticancer drug, but
like many anticancer drugs, one of its major side effects is diarrhea. The administration
of PHY906 with irinotecan in a mouse model of colon cancer resulted in a synergistic
reduction in tumor burden, enhanced maintenance of body weight, and stem cell regeneration
in the intestinal mucosa [47]. PHY906 did not protect against the initial DNA damage and apoptosis triggered by
irinotecan in the intestine, but by 4 days after irinotecan treatment, PHY906 had
restored the intestinal epithelium by promoting the regeneration of intestinal progenitor
or stem cells via several Wnt signaling components. PHY906 also potentiated Wnt3a
activity in human embryonic kidney-293 cells. Furthermore, PHY906 exhibited anti-inflammatory
effects in mice by decreasing infiltration by neutrophils and macrophages, upregulating
tumor necrosis factor-α expression in the intestine, and increasing proinflammatory cytokine concentrations
in plasma. Chemical constituents of PHY906 potently inhibited nuclear factor κB (NFκB), cyclooxygenase-2, and inducible nitric oxide synthase. Our results show that the
herbal medicine PHY906 can counteract the toxicity of irinotecan via several mechanisms
that act simultaneously.
Effects of Natural Products On Cancer Stem-Like Cells
Effects of Natural Products On Cancer Stem-Like Cells
Cancer stem cells give rise to the tumor bulk through continuous self-renewal and
differentiation. Understanding the mechanisms that regulate self-renewal of cancer
stem-like cells and the points at which there is potential for intervention by natural
products is of greatest importance in the discovery of anticancer drugs targeting
cancer stem cells ([Table 1]). Relapse in cancer is mostly due to the proliferation of cancer stem cells that
could not be eliminated by standard chemotherapy.
Table 1 Effects of natural products towards cancer stem-like cells.
|
Natural product
|
Cell type
|
Effect
|
Reference
|
|
Curcumin, sulforaphane, soy isoflavone, epigallocatechin-3-gallate, resveratrol, lycopene,
piperine, vitamin D(3)
|
breast cancer stem-like cells
|
signal pathways (Wnt/β-catenin, Hedgehog, Notch) ↓
|
[48]
|
|
Curcumin
|
C6 brain tumor cells (side population)
|
side population ↓
|
[58]
|
|
Resveratrol
|
mammospheres
|
apoptosis, DAPK2, BNIP3 and lipid synthesis ↓, fatty acid synthase ↓
|
[52]
|
|
Resveratrol
|
medulloblastoma stem-like cells
|
proliferation ↓, radiosensitivity
|
[57]
|
|
Z-guggosterole
|
murine breast cancer metastases
|
CD44 positive cells ↓, apoptosis markers (caspase-3, ceramide) ↑
|
[49]
|
|
Pomegranate extract
|
murine WA4 MMTV-Wnt-1 breast tumors
|
G0/G1 cell cycle arrest, caspase-3, apoptosis
|
[50]
|
|
Ellagic acid, ursolic acid luteolin sulphoraphane
|
breast cancer stem-like cells (xenograft tumors)
|
proliferation and viability ↓, Wnt/β-catenin pathway ↓, aldehyde dehydrogenase-positive cells ↓
|
[51]
|
|
All-trans retinoic acid
|
breast cancer stem-like cells
|
G0/G1 cell cycle arrest, cellular differentiation
|
[53]
|
|
Clostridium perfringens enterotoxin
|
ovarian cancer stem-like cells in vitro and in vivo
|
proliferation of CD44- and claudin-4 positive cells ↓
|
[54]
|
|
Lupeol
|
CD133 positive hepatocellular carcinoma cells
|
CD133 expression ↓, tumorigenicity in nude mice ↓
|
[55]
|
|
Gossypol
|
prostate cancer in vitro and in vivo
|
DNA damage, p53 activation, apoptosis
|
[56]
|
|
Eckol
|
glioma stem-like cells in vitro and in vivo
|
CD133 cell population ↓, PI3K/AKT and Ras/Raf/Erk signaling pathways ↓, anchorage-independent
growth ↓, xenograft growth ↓, drug and radio resistance ↓
|
[59]
|
|
Parthenolide
|
myeloma
|
preferential toxicity to stem-like cells
|
[60]
|
|
Grapholide
|
stem-like cells
|
|
|
|
Cantharidin
|
primary acute myeloic leukemia stem and progenitor cells in vitro
|
cytotoxicity
|
[61]
|
|
Rakicidin A
|
hypoxia-adapted chromic myeloic leukemia stem-like cells
|
apoptosis, synergistic interaction with imatinib
|
[62]
|
Breast cancer
Several dietary compounds including curcumin, sulforaphane, soy isoflavone, epigallocatechin-3-gallate,
resveratrol, lycopene, piperine, and vitamin D(3) have been identified as having a
direct or indirect effect on self-renewal pathways (Wnt/β-catenin, Hedgehog and Notch) [48]. Curcumin and piperine have been demonstrated to target breast cancer stem cells.
Sulforaphane has been reported to inhibit growth of pancreatic tumor-initiating cells
as well as breast cancer stem cells. These studies provide a basis for preclinical
and clinical evaluation of dietary compounds for chemoprevention against cancer stem
cells.
The cancer stem cells play a critical role in both initiation and relapse, as they
are resistant to most cytotoxic agents and are able to proliferate indefinitely. Krishnamurthy
et al. [49] found that the bile acid sodium deoxycholate increased the number of intestinal
metastases generated from murine mammacarcinoma 4 T1 tumors. The metastatic nodes
contained slowly dividing cancer cells in the immediate vicinity of newly formed blood
vessels. These cells were positive for CD44, a biomarker for breast cancer stem cells.
Deoxycholate promoted survival of CD44+ cells and concurrently reduced levels of activated caspase-3 as well as ceramide,
a sphingolipid that induces apoptosis in 4 T1 cells.
Z-guggulsterone, an antagonist of the farnesoid-X-receptor, obliterated this anti-apoptotic effect,
indicating that deoxycholate increased cell survival via the farnesoid-X-receptor.
Deoxycholate also increased expression of the gene coding for the vascular endothelial
growth factor receptor 2 (Flk-1) in tumor cells, suggesting that deoxycholate enhances
the initial growth of secondary tumors adjacent to blood vessels. The authors concluded
that treatment with Z-guggulsterone and/or vascular endothelial growth factor receptor
2/Flk-1 antagonists may be a promising strategy for reducing breast cancer metastasis.
Pomegranate (Punica granatum L.) is another natural substance known to possess anticancer activities. The effects
of a standardized pomegranate extract on a mouse mammary cancer cell line (designated
WA4) derived from mouse MMTV-Wnt-1 mammary tumors were examined [50]. The WA4 cell line has been previously characterized, and it has been found that
the majority of WA4 cells possess stem cell characteristics. The pomegranate extract
inhibited the proliferation of WA4 cells in a time- and concentration-dependent manner
due to an arrest of cell cycle progression in the G0/G1 phase. Pomegranate extract
was also cytotoxic to quiescent WA4 cells and resulted in an increase in caspase-3
enzyme activity by induction of the apoptotic pathway. The phytochemicals ellagic
acid, ursolic acid, and luteolin, derived from pomegranate extract, caused a time-
and concentration-dependent reduction in cell proliferation and viability, suggesting
that they contribute to the inhibitory effect of the extract. The authors conclude
that the effects observed in their investigation are due to inhibition of cancer stem-like
cells.
Sulforaphane is a natural compound derived from broccoli and broccoli sprouts. Sulforaphane decreased
the aldehyde dehydrogenase-positive subpopulation of cells and reduced the size and
number of primary mammospheres in tumor xenografts in a nonobese diabetic/severe combined
immunodeficient (NOD/SCID) mouse model [51]. Sulforaphane eliminated breast cancer stem-like cells in vivo, thereby abrogating tumor growth after reimplantation of primary tumor cells into
mice. Western blotting analysis and a β-catenin reporter assay showed that sulforaphane downregulated the Wnt/β-catenin self-renewal pathway. These findings support the use of sulforaphane for
chemoprevention against breast cancer stem cells.
Resveratrol significantly reduced cell viability and mammosphere formation and subsequently induced
apoptosis in cancer stem-like cells [52]. The inhibitory effect of resveratrol was accompanied by a significant reduction
in lipid synthesis caused by the downregulation of the fatty acid synthase (FAS) gene followed by the upregulation of the pro-apoptotic genes DAPK2 and BNIP3. Resveratrol-induced
activation of the apoptotic pathway in cancer stem-like cells was suppressed by TOFA
and by fumonisin B1, suggesting that resveratrol induces apoptosis through the modulation
of fatty acid acid-mediated cell survival signaling. Resveratrol was able to significantly
suppress the growth of cancer stem-like cells in an animal xenograft model with no
apparent toxicity.
All-trans retinoic acid (ATRA), a form of vitamin A, has also shown potential as an anticancer agent. The
inhibitory effect of ATRA stealth liposomes was more potent in CD44+/CD24− cancer stem cells than in cancer cells without this phenotype [53]. Its mechanisms include arrest of breast cancer stem cells at the G0/G1 phase and
induction of differentiation. Formation and growth of xenografted tumors were significantly
inhibited by ATRA stealth liposomes. Combination therapy of ATRA stealth liposomes
with vinorelbine stealth liposomes produced the strongest inhibitory effect on the
tumor relapse model compared to free ATRA, and vinorelbine stealth liposomes.
Ovarian cancer
During the characterization of CD44+ ovarian cancer stem cells, Casagrande et al. [54] found a high rate of expression of the genes encoding claudin-4. CD44+ ovarian cancer stem cells expressed the claudin-4 gene at significantly higher levels
than matched autologous CD44- ovarian cancer cells. Because claudin-4, a tight junction protein, is a natural and
high-affinity receptor for Clostridium perfringens enterotoxin, the authors investigated the sensitivity of ovarian cancer stem cells
to Clostridium perfringens enterotoxin treatment in vitro and in vivo. Small-interfering RNA-mediated knockdown of claudin-3/-4 expression in CD44+ cancer stem cells significantly protected cancer stem cells from Clostridium perfringens enterotoxin-induced cytotoxicity. Multiple intraperitoneal administrations of sublethal
concentrations of Clostridium perfringens enterotoxin in mice harboring xenografts of chemotherapy-resistant CD44+ ovarian cancer stem cells had a significant inhibitory effect on tumor progression.
In fact, all treated animals were either cured or achieved long-term survival. Hence,
Clostridium perfringens enterotoxin may represent an unconventional but potentially highly effective strategy
to eradicate chemotherapy-resistant cancer stem cells.
Liver cancer
Lupeol (lup-20(29)-en-3β-ol) is a triterpene found in fruits and vegetables that inhibited the self-renewal
ability of liver tumor-initiating cells in both hepatocellular carcinoma (HCC) cell
lines and clinical HCC samples [55]. Furthermore, lupeol inhibited tumorigenicity in vivo in nude mice and downregulated CD133 expression. In addition, lupeol sensitized HCC
cells to chemotherapeutic agents through the phosphatase and tensin homolog (PTEN)-Akt-ABCG2
pathway. PTEN plays a crucial role in the self-renewal and drug resistance of liver
tumor-initiating cells T-ICs; downregulation of PTEN via a lentiviral-based approach
reversed the effect of lupeol on liver T-ICs. Using an in vivo drug-resistant HCC tumor model, lupeol dramatically decreased the tumor volumes of
MHCC-LM3 HCC cell line-derived xenografts. Lupeol exerted a synergistic effect when
combined with established chemotherapeutic drugs (cisplatin, doxorubicin) without
any adverse effects on body weight.
Prostate cancer
Gossypol reduced the viability of tumor initiating CD44+ prostate cancer cell lines in vitro. Additionally, gossypol inhibited prostate tumor-initiating cell growth in a xenograft
model. The decrease in viability was associated with increased DNA damage, activation
of p53, and induction of apoptosis [56].
Brain cancer
Lu et al. [57] isolated cancer stem-like cells from medulloblastoma patient samples and cultured
them as three-dimensional spheroids. They displayed enhanced self-renewal and expressed
stem-like genes (Oct-4, Nanog, Nestin, Musashi-1) and anti-apoptotic genes (Bcl-2,
Bcl-xL). These spheroids also showed significant resistance to radiotherapy as compared
to the parental medulloblastoma cells in 2D culture. Resveratrol effectively inhibited
the proliferation of stem-like medulloblastoma cells and significantly enhanced radiosensitivity.
Fong et al. [58] treated rat C6 glioma cells with curcumin and observed a decrease in a specific
side population of C6 cells that retained less Hoechst 33342 dye after daily curcumin
treatment. Since the side population exhibited stem-like cell markers, curcumin may
be a dietary phytochemical with the potential to target cancer stem-like cells.
Treatment of spheroid-forming glioma cells with Eckol, a phlorotannin compound, effectively
decreased sphere formation as well as the CD133+ cell population, and blocked both phosphoinositide 3-kinase-Akt and Ras-Raf-1-Erk
signaling pathways. Eckol treatment suppressed expression of the glioma stem-like
cell markers and the self-renewal-related proteins without causing cell death. Moreover,
Eckol significantly attenuated anchorage-independent growth on soft agar and tumor
formation in xenograft mice. Importantly, Eckol treatment effectively reduced the
resistance of glioma stem-like cells to ionizing radiation and temozolomide [59].
Multiple myeloma
Gunn et al. [60] tested two natural product inhibitors of NFκB, parthenolide and andrographolide, in multiple myeloma stem-like cells. Both compounds
demonstrated preferential toxicity toward cancer stem-like cells as compared to non-tumorigenic
multiple myeloma cells. Coculture with bone marrow stromal cells in their 3D reconstructed
bone marrow model abrogated andrographolide activity while having no effect on parthenolide
cytoxicity.
Leukemia
Cantharidin targeted primary acute myeloic leukemia stem and progenitor cells in vitro in contrast to conventional chemotherapeutic agents (e.g., Ara-C and daunorubicin)
that mainly targeted more differentiated leukemic cells [61]. Because of concentration-limiting toxicity in vivo, neither cantharidin nor norcantharidin proved therapeutically beneficial in acute
myeloic leukemia xenograft models as a single agent. However, their potent in vitro activity against leukemic stem-like cells and pathway targeting may still be exploited
clinically with a new generation of cantharidin derivatives.
Treatment with Abl tyrosine kinase inhibitors drastically improves the prognosis of
chronic myeloic leukemia patients. However, quiescent CML cells are insensitive to
tyrosine kinase inhibitors and can cause relapse. The hypoxic conditions in the bone
marrow allow leukemic cells that reside there to become insensitive to cell death
stimuli. Takeuchi et al. [62] described that rakicidin A, a natural product produced by a Micromonospora strain, induced cell death in hypoxia-adapted chronic myeloic leukemia cells with
stem cell-like characteristics. Apoptosis was induced by both caspase-dependent and
-independent pathways. Treatment with rakicidin A in combination with the tyrosine
kinase inhibitor imatinib resulted in synergistic cytotoxicity.
Conclusion and Perspectives
Conclusion and Perspectives
Many natural products have been described that affect cellular pathways relevant for
stem cells and cancer stem-like cells, but have not yet been tested in these cell
types specifically. Hence, there are a number of natural compounds to be explored
in cancer stem cell biology. Some examples are:
-
Natural small molecules such as vitamin A and retinoic acid inhibit cancer cell growth
by induction of differentiation rather than by cytotoxicity [39], [64], [65], [66], [67]. The potency of retinoid acids for stem-like cell treatment is promising and needs
to be further explored.
-
Numerous phytochemicals that are able to inhibit ATP-binding cassette (ABC)-mediated
multidrug resistance (for review see [68]). ABC-transporters such as BCRP are important markers of cancer side-population
cells which resemble cancer stem-like cells. ABC transporters contribute to the drug-resistant
phenotype of cancer stem-like cells.
-
Phytochemicals that inhibit NF-κB [69], [70]. NF-κB is a transcription factor that mediates resistance towards apoptotic stimuli. It
promotes tumor progression and drug- and radio-resistance. For example, parthenolide
is a well-known NF-κB inhibitor that inhibits stem-like breast cancer cells [71]. Another promising drug may be the antimalarial artesunate, which is also active
against cancer cells. It has been shown to inhibit growth of rat embryo (stem) cells
[72], to induce cellular differentiation [73] and to inhibit NF-κB [74], [75].
-
Phytochemicals that inhibit signaling pathways of stem cells and cancer stem-like
cells. Certain pathways trigger the self-renewal of stem and stem-like cells. Their
inhibition suppresses self-renewal and induces cellular differentiation. Interesting
examples are:
-
Cyclopamine from Veratrum californicum, which inhibited growth of medulloblastoma cells and induced a loss of neuronal stem
cell-like character by neuronal differentiation in a mouse model [76].
-
ECGC from green tea (Camelia sinensis), which inhibits Wnt/β-catenin signaling. This pathway is involved in various developmental processes, including
growth inhibition in breast cancer cells [77]. Comparable effects have been found for vitamin D on Wnt/β-catenin signaling in colorectal cancer cells [78]. Artesunate also inhibits the Wnt/β-catenin signaling [79], [80].
-
Resveratrol, a polyphenol in red grapes and other fruits, that downregulates the expression
of Notch [81]. The Notch signaling pathway is also involved in developmental processes and regulates
tumor self-renewal in medulloblastoma [82].
Natural products may gain considerable importance in stem cell biology for both tissue
regeneration and for protection of normal tissue against the side effects of cancer
chemo- and radiotherapy ([Fig. 1]). Established anticancer drugs kill the bulk of differentiated cancer cells, but
leave undifferentiated and quiescent cancer stem-like cells untouched. This population
of cancer stem-like cells can give rise to tumor relapse even after chemotherapy has
achieved complete clinical remission of tumors [63]. The results obtained with natural products in the past few years are promising,
as several compounds have been reported to attack cancer stem-like cells. Furthermore,
many of these natural products also improve the efficacy of chemo- and radiotherapy,
making them ideal partners for combination therapy regimens. The field of treating
cancer stem-like cells with natural products is still in its infancy, but it can be
expected to develop into a rapidly growing area of research.
Fig. 1 Therapeutic potential of natural products towards stem cells.
Acknowledgements
We thank Karen Duffy (Cornell University, Ithaca, NY, USA) for reading and commenting
on the manuscript
