Download Discussing the Evolving Role of Cancer Stem Cells

EXPERT PANEL
LOCAL INSIGHTS
Discussing the Evolving Role
of Cancer Stem Cells
Lee M. Ellis, MD
Professor of Surgery, Department
of Surgical Oncology
William C. Liedtke Jr. Chair
in Cancer Research
The University of Texas
MD Anderson Cancer Center
Houston, Texas
INTRODUCTION
Besides understanding what gives rise to cancer, the major clinical
challenges in managing these diseases include treatment failure or
resistance, recurrence, and metastasis. Over the past several decades,
increasing evidence has mounted for the existence of cancer stem cells
(CSCs) and their involvement in these challenges.1,2
Like normal stem cells, CSCs have the potential to self-renew and
differentiate into progenitor cells, which further differentiate into tumor
cells.3 Normal stem cells are, however, under strict controls to limit these
potentials. In general, they remain dormant except during normal tissue
development or in response to injury. While CSCs share some of the
main features of normal stem cells, the processes of proliferation and
genetic repair are no longer subject to the usual control mechanisms.
This creates a situation in which the CSC activation and differentiation
potentials are altered.3,4
Self-renewing, tumorigenic CSCs have been found in multiple tumor
types and are usually resistant to standard therapies. These properties of
CSCs make them suited to serve as the putative source of tumor origin
as well as the source of tumor heterogeneity, recurrence, therapeutic
resistance, and metastasis.2,4
With the ultimate goal of developing more effective approaches to
cancer management, researchers are endeavoring to gain a complete
understanding of CSC biology. They are working to find ways to
identify CSCs and to develop strategies that inhibit their stem-cell–
like properties and related signaling pathways, or that destroy them
directly. The hope is that these approaches will help advance long-term
cancer management. Local Insights has brought together clinicians
and researchers to discuss the role of CSCs in cancer development and
progression, and potential treatment approaches to interfere in these roles.
©2015 Boston Biomedical, Cambridge, MA 02139. All rights reserved. Printed in USA/December 2015.
Jenny C. Chang, MD
Emily Herrmann Chair in Cancer
Research & Division Chief, Cancer Center
Professor of Cancer, Institute for
Academic Medicine
Director
Houston Methodist Cancer Center
Houston, Texas
Sendurai A. Mani, PhD
Associate Professor, Department of
Translational Molecular Pathology
Co-Director, Center for Stem Cells &
Developmental Biology
The University of Texas
MD Anderson Cancer Center
Houston, Texas
Supported by
EDU-NPS-0063
LOCAL INSIGHTS: Discussing the Evolving Role of Cancer Stem Cells
SECTION 1. What Are CSCs?
Over 150 years ago, Virchow and others proposed that
tumors, like other tissues, may arise from a subpopulation
of cells with stem-cell–like properties.5 Only within the
last 20 years, however, has proof of the existence of CSCs
been demonstrated.2 In 2006, the American Association
for Cancer Research Workshop on Cancer Stem Cells
established the definition of CSCs as “cells within a
tumor that possess the capacity to self-renew and to give
rise to the heterogeneous lineages of cancer cells that
comprise the tumor.” CSCs are sometimes referred to in
the literature as “tumor-initiating cells” or “tumorigenic
cells.”6 Today, CSCs have been identified in numerous
hematologic and solid malignancies, including leukemia,
lung cancer, colon cancer, prostate cancer, breast cancer,
ovarian cancer, brain cancer, and melanoma.2,7 CSCs
interact with their tumor microenvironment to maintain
stem-cell–like properties, or “stemness,” that enable them
to produce a variety of differentiated cells that compose the
majority of the tumor.3 (See The Microenvironment on
page 6.) A common misconception is that all CSCs arise
from mutated normal stem cells, but some CSCs may arise
from progenitor cells when a mutation endows these cells
with the capacity for self-renewal, which is normally only
the case with stem cells (Figure 1).5,8
The Stochastic and Hierarchy Models
of Tumor Heterogeneity
Figure 1. Normal stem cell and cancer stem cells (CSCs) devel-
Two main models exist to explain the development of
heterogeneity within tumors: the clonal evolution or
stochastic model and the CSC or hierarchy model.9 Longheld theories about cancer, such as the stochastic model,
posited that all cells within a tumor had essentially the same
genetic changes and exhibited the same phenotype. Because
it was thought that tumors were mostly homogeneous
and that all tumor cells possessed tumor-initiating
activity, cancer research was conducted on whole tumor
populations, and treatments were developed keeping
in mind the eradication of the tumor bulk.10 In recent
decades, observations of tumor heterogeneity and
evidence for the existence of CSCs have presented a
challenge to such models. The hierarchy model holds
that minority subpopulations of biologically and
functionally distinct cells have the capacity to initiate
tumor growth and foster other tumor properties such
as progression and aggressiveness.4 Furthermore,
these tumor-initiating cells can be identified and
isolated from the bulk tumor.10
While these models appear mutually exclusive,
some have proposed that merging the models
may actually provide a fuller explanation of tumor
maintenance and progression. In fact, evidence
indicates that aspects of the two models may
describe tumor organization at different stages or
under varying microenvironmental conditions. Such
a unified model may account for the variation in
the frequency, genotype, and phenotype of CSCs
between patients and tumors.2,11,12
opment. Normal stem cells self-renew (curved green arrow) and
differentiate into progenitor cells, which then differentiate into
mature tissue. CSCs develop via mutation of normal stem cells or
progenitor cells (dashed arrow), and can self-renew and differentiate
into heterogeneous tumors.8
2
CSC Properties
Within both primary and metastasized tumors,
cell subpopulations can differ on the basis of such
factors as morphology, expression of surface antigens,
specific alterations of the genome, and patterns of gene
expression.13-15 Likewise, CSCs are heterogeneous with
varying degrees of self-renewal capacity, development
potential, and expression of cellular markers. Like normal
stem cells, CSCs exist in a hierarchy.5 Their capacity for
self-renewal and differentiation places CSCs at the top of a
cellular hierarchy from which all other cells within a tumor
are derived. In this light, tumor origin and development
may be likened to normal tissue development wherein
stem cells differentiate into committed progenitor cells,
and these further differentiate into the terminal cell
types that constitute the bulk of the tissue.16 The analogy
to normal stem cells and normal tissue development
weakens, however, in light of the fact that CSCs are
cancerous. As noted, they are not subject to normal cell
control mechanisms. Instead, CSCs undergo uncontrolled
replication and differentiation, resulting in progenitor and
terminal cell populations with altered molecular and cellular
phenotypes.2,4,13,17 A listing of the characteristics of normal
stem cells and CSCs can be seen in Table 1.3,4,13,18
Using glioma stem cells, research has shown that CSCs
can divide symmetrically, producing new CSC progeny,
or asymmetrically, producing non-stem cell and stem cell
progeny. Intratumoral heterogeneity likely derives from
asymmetrical division and differentiation of CSCs.19
Over time, unrestrained differentiation and proliferation
produces the heterogeneous populations of primary and
metastatic tumor cells that contribute to tumor properties
such as recurrence, resistance to therapy, and metastasis.13
Table 1. C
haracteristics of Normal Stem Cells and
Cancer Stem Cells3,4,13,18
Normal Stem Cells
CSCs
Characteristic
Self-renewal
Differentiation
Plasticity
Quiescence
✓
✓
—a
✓✓
✓✓
✓✓
✓
✓
a
Injury or experimental manipulation may induce plasticity in differentiated
normal stem cell progeny
Treatment Resistance
CSCs tend to be resistant to chemotherapeutic agents and
radiation.20 The resistance of CSCs to standard therapies
is due, at least in part, to their low rate of proliferation.20
Following treatment, the remaining CSCs can repopulate
the tumor, causing treatment failures and disease relapses.13
Another feature of CSC biology and other tumor cells is that
they possess inherent genetic and epigenetic instability. This
instability is the foundation of tumorigenesis and tumor
cell heterogeneity. It also implies that CSCs and non-CSCs
within a tumor can assume new phenotypes at different stages
during the tumor life cycle. Thus, the cells are moving targets
with the potential to develop treatment resistance over time.6
CONSULTANT COMMENTARY
Dr Chang: Simply, CSCs are a subpopulation of
tumor cells that have repopulating potential, can
self-renew, and are treatment resistant.
Dr Mani: Cancer is made up of various cell types.
Two important ones are differentiated cancer cells
and CSCs. CSCs have been shown to be responsible
for tumor relapse, acquisition of resistance to
chemotherapy, and progression of disease to
metastasis. Clearly, our group at MD Anderson
and others have demonstrated that CSCs can be
generated not only from the transformation of normal
stem cells but also from the differentiated cancer
cells by the activation of a latent embryonic program
known as epithelial-mesenchymal transition.
Dr Chang: In my mind, the hierarchical/CSC and
the clonal/stochastic theories are intertwined.
They are not mutually exclusive. Even in clonal
evolution, the ancestral clones appear to have
more metastatic potential because they essentially
have stem-cell–like properties.
Dr Ellis: We have the stochastic model, which is
random, and the hierarchical model. The truth is
somewhere between. Let’s recognize that there’s
a whole spectrum of CSCs that differentiate
themselves. The two extremes are probably stable
and in between they’re probably very plastic.
3
LOCAL INSIGHTS: Discussing the Evolving Role of Cancer Stem Cells
SECTION 2. The Importance of CSCs
Currently, conventional cancer treatments are intended
to reduce bulk tumor burden by surgical resection9,11,21
or by killing mature, rapidly proliferating tumor.8,20
However, these treatments usually have little effect
on CSCs, which are only a small portion of the total
tumor mass.13 As a result, patients often experience
initial successful treatment responses that later give
way to tumor recurrence, progressive disease, therapy
resistance, and metastases.13
chemotherapeutic agents caused the emergence of a
drug-resistant subpopulation of cells expressing CSC
markers CD24, CD133, CD44, and CXCR4.1
•In a mouse breast cancer model, ionizing radiation reduced
the overall tumor volume but induced an increase in stem
cell behavior as well as the expression of stemness genes
and proteins. In addition, radiation treatment correlated
with an approximate 50% increase in the nodular area of
lung metastases compared to the controls.23
•Other research found that irradiation of breast cancer
cells in vitro induced expression of Notch receptor
family members that are known to be important in the
self-renewal of breast and glioma CSCs.24
Research has also indicated that the presence of CSCs
within a tumor is associated with worse overall prognosis
in many cases.4,13 Functionally, CSCs have been found to
be pivotal in such processes as angiogenesis, resistance to
apoptosis, and cell migration and invasion.13
As CSC biology is elucidated, increasing evidence
suggests that effective tumor eradication will require
the development of therapeutic regimens directed against
CSCs, their underlying signaling pathways, and non-CSC
tumor cells. As much as possible, these therapies must still
spare normal stem cells and healthy tissue.2,6
Although chemotherapy and radiation
therapy may eradicate much of the tumor
bulk, theoretically, CSCs may remain, giving
rise to treatment resistance.13
In addition to resistance to many conventional
treatment modalities, emerging evidence suggests that some
current cancer therapies may actually enrich subpopulations
of drug-resistant CSCs, increasing their numbers and
their stem cell activity, and possibly, accelerating disease
progression (Figure 2).3,5,22 For example:
•Using human colon cancer cell lines, researchers found
that treatment with two of the more commonly used
Figure 2. Cancer stem cells (CSCs) and
treatment resistance. Although current therapeutic regimens kill a significant amount of
tumor cells, experimental evidence suggests
that these regimens enrich therapy-resistant
CSCs that are highly tumorigenic and may
lead to tumor relapse.3,5,22
4
A Brief History of CSC Research
The concept of a rare population of stem cells as the
origin of cancer was first proposed over 150 years ago
by Virchow and Cohnheim.1 In the 1970s, researchers
advanced the theory that tissue-specific stem cells might
be the cells of origin for specific cancers, but technical
advances were needed to provide experimental proof
to support the CSC hypothesis.1,2
group used cellular marking studies to show that the
leukemic CSCs resembled normal stem cells in their
degree of heterogeneity and their varying capacity for
self-renewal.4
Other researchers produced corroborative findings in
breast CSCs.5 Such findings have led to the hypothesis
that CSCs are part of a developmental hierarchy that
functions much like normal stem cells and that CSCs
derive from malignant transformation of normal stem
cells.2 Subsequent research has continued to find CSCs
in other tumor types, including those in the brain,
prostate, and lung, as well as in multiple myeloma.2
Bonnet and Dick provided such evidence in 1997 by
demonstrating the existence of “leukemic-initiating
cells” that possessed differentiative, proliferative,
and self-renewal capacities consistent with what was
expected of CSCs.3 In 2004, researchers in the same
1. Huntly BJ, Gilliland DG. Leukaemia stem cells and the evolution of cancer-stem-cell research. Nat Rev Cancer. 2005;5(4):311-321.
2. Wicha MS, Liu S, Dontu G. Cancer stem cells: an old idea—a paradigm shift. Cancer Res. 2006;66(4):1883-1890; discussion 1895-1886.
3. Bonnet D, Dick JE. Human acute myeloid leukemia is organized as a hierarchy that originates from a primitive hematopoietic cell. Nat
Med. 1997;3(7):730-737.
4. Hope KJ, Jin L, Dick JE. Acute myeloid leukemia originates from a hierarchy of leukemic stem cell classes that differ in self-renewal
capacity. Nat Immunol. 2004;5(7):738-743.
5. Al-Hajj M, Wicha MS, Benito-Hernandez A, Morrison SJ, Clarke MF. Prospective identification of tumorigenic breast cancer cells. Proc
Natl Acad Sci U S A. 2003;100(7):3983-3988.
CONSULTANT COMMENTARY
Dr Mani: A tumor is not static tissue. While you’re
treating a tumor today, it’s evolving. CSCs appear to
play a significant role in that process. In fact, CSCs have
been shown to have a role in all types of cancer, both
solid and hematologic.
Dr Mani: Because of its chemical nature, chemotherapy
can promote inflammation, which can induce plasticity.
It’s as though chemotherapy is “angering” the CSCs.
Alternatively, chemotherapy could be killing differentiated
cancer cells and enriching CSC populations.
Dr Ellis: Theoretically, CSCs proliferate at a lower
rate than more differentiated tumor cells. CSCs are
hypothesized to mediate resistance to chemotherapies
and, perhaps, targeted therapies. If, indeed, a
subpopulation of CSCs exists, and they can be targeted
or eliminated, the tumor should lose its “source cell.”
This would allow better control of tumor growth or
apoptosis. However, this is easier said than done.
Identifying the multiple drivers of “CSC-ness” may
provide a new opportunity for therapeutic targeting.
5
LOCAL INSIGHTS: Discussing the Evolving Role of Cancer Stem Cells
SECTION 3.
The Role of the Microenvironment and CSC
Signaling Pathways
Signaling Pathways
CSCs grow and develop in a microenvironment that supports
their survival, propagation, and differentiation into the cells
that define the tumor mass.14 In addition, CSC development
is driven by a number of key signaling pathways and
elements.2 Dysregulation of the pathways and elements plays
an important role in the ability of CSCs to self-renew and
differentiate. Depending on the pathway involved, CSCs may
initiate cancer formation or cause tumor recurrence.2
Signaling pathways are key components in all cells.
They stimulate a wide variety of cell processes––from
cell growth, proliferation, and differentiation to invasion
and apoptosis. Well-known internal signals or pathways
that function in normal stem cell niches include the
Wnt, Notch, Hedgehog (Hh), and Janus kinase/signal
transducer and activator of transcription (JAK/STAT)
pathways.12,26 Several intracellular signaling pathways
may be altered in the process of malignant transformation
of stem cells (Figure 4).2 For example:
•The evolutionarily conserved Wnt family of proteins are
cysteine-rich, secreted glycoproteins that control tissue
homeostasis, and regulates diverse processes during
development. Wnt pathway dysregulation has been
identified in several hereditary diseases and is associated
with intestinal cancers.27
The Microenvironment
CSCs exist within a microenvironment of surrounding
vasculature, stromal cells, immune cells, and secreted
factors produced by these cells (Figure 3).5,14 These create
a niche wherein the CSCs can survive and thrive in order
to propagate and differentiate into the cells that make
up the tumor mass. In essence, the niche is a regulatory
microenvironment that nurtures the stem-cell–like
characteristics of CSCs so that they can generate,
or regenerate, the tumor bulk and maintain their
self-renewing potential.
The microenvironments that maintain normal
stem cells, serve as a reserve for repopulating cell
populations lost due to aging or damage. In the
case of cancer development, these normal stem
cell microenvironments may, however, become
dysfunctional, hijacking their function to maintain
CSCs. These CSCs may function as a tumor-cell
reserve to foster recurrence following treatment.
Evidence of this has been shown in brain tumors
wherein CSCs were found with neighboring
endothelial cells. Such an association resembles
the usual niche for normal stem cells, which are
the likely cells of origin for CSCs in these types
of tumors. Furthermore, higher irradiation of a
potential neural CSC niche improved survival
among glioma patients.9,12
Intracellular and intercellular signals operate
within CSC microenvironments and support
CSC activities. The internal signals include
molecular pathways that regulate stemness, whereas Figure 3. Interaction of cancer stem cells (CSCs) with their
microenvironment. CSCs exist and thrive within a microenvironment
extracellular signals consist of cells designed to
containing stromal cells, immune cells, neighboring vasculature, and
anchor CSCs within the microenvironment, and cell factors secreted by these cells. These components create a niche
receptors and secreted factors that are necessary for where CSCs survive and propagate into a heterogeneous tumor mass.
maintaining CSCs in their quiescent state.25
Courtesy of Boston Biomedical.5,14
6
•The Notch pathway has crucial roles in stem cell control
and cell-fate determination.28 Research has found that a
signature of Notch pathway in CSCs identified patients
with poorly differentiated lung adenocarcinoma, and
was prognostic for poorer overall survival. By inhibiting
the Notch pathway, CSCs were prevented from forming
tumors when implanted into mice.28
•The Hh protein family members turn on the genes that
regulate the cell cycle and determine cell fate. They
are also known to be key regulators of carcinogenesis.
Hh and downstream factors have been shown to have
significant roles in pancreatic cancer, glioma, and
basal cell carcinoma. Inhibition of the Hh pathway in
pancreatic cancer depressed the self-renewal of CSCs
and impaired their resistance to chemotherapy.7,16,29
•The Hippo pathway and its related mediators Yes/Yap
regulate several tumor suppressor genes to control cellular
processes such as survival, proliferation, differentiation,
apoptosis, and stem or progenitor cell expansion.1,30
Dysregulation of the Hippo pathway has been identified in
multiple cancers including liver, lung, colorectal, ovarian,
and prostate.30 Researchers also found that the expression
levels of Yes/Yap genes were prognostic for survival in
patients receiving certain types of chemotherapy.1
•NANOG is a transcription factor involved in the selfrenewal and maintenance of pluripotency in normal
stem cells. Experimental inhibition of NANOG or
related transcription factors have been shown to
decrease stem-cell–like activities in breast cancer,
colorectal cancer, prostate cancer, and melanoma.31,32
Several specific signaling pathways––
JAK/STAT, Wnt, Notch, Hh and
Hippo––have been shown to be
involved in the induction and
maintenance of CSC stemness.2,31,35-40
•The STAT family of transcriptional factors cooperates
with NANOG to transcribe stemness genes that are
required for modulating pluripotency.33 The STATS are
upstream signals activated by interleukin-6 (IL-6).34
Activated STAT3 has been found in leukemia,
squamous cell carcinoma of the head and neck,
multiple myeloma, breast cancer, and prostate
cancer.26 Blockade of the STAT3 signaling pathway
has been shown to inhibit the clonogenic and
tumorigenic potential of CSCs in prostate cancer.26
In addition, it has been shown that blockade of
STAT3 activity inhibits both tumor growth and
tumor-initiating potential in colon CSCs.34
Cancer-associated cells in the microenvironment
may secrete growth factors and cytokines to support
CSCs.11 Examples of these include cytokines such
as stromal cell-derived factor-1, IL-6, and IL-8, all
of which function to regulate CSC activity and are
thought to play roles in treatment resistance.11 For
example, it has been shown that myofibroblasts
within colorectal tumor-associated stroma
secrete hepatocyte growth factor that maintains
CSC function by activating the Wnt pathway.9,40
Furthermore, research on glioblastomas has
found that in some tumors there is a bidirectional
Figure 4. Key signaling pathways in cancer stem cell (CSC)
relationship between the CSCs and the local
development. Dysfunction of several developmental and
microenvironment. These data suggest that elements
stemness signaling pathways contribute to the generation of
of the microenvironment affect the cellular behavior
CSCs and their ability to initiate cancer formation and cause
of CSCs. Conversely, CSCs may be able to modify
tumor recurrence.2
their microenvironment.40
7
LOCAL INSIGHTS: Discussing the Evolving Role of Cancer Stem Cells
Additionally, direct, cellular interactions between CSCs
and other cells of the microenvironment function to
preserve CSCs and affect their functions. In ductal breast
carcinoma, elevated levels of the integrin molecule, focal
adhesion kinase (FAK), have been linked with an increased
invasive phenotype, and ablation of FAK function reduced
the number of CSCs in a mouse breast cancer model.
Many researchers have found further links between FAK
signaling and breast CSC maintenance, tumor progression,
and metastasis.11
CONSULTANT COMMENTARY
Dr Chang: Some cells have adaptive response to
different stresses. Under different conditions, these
cells will have different properties. We believe that the
properties of CSCs can change depending upon the
stresses, like chemotherapy and the microenvironment.
Dr Chang: From the clinical point of view, this is
absolutely true. We are just starting to understand what
triggers CSCs. We are developing therapies that target
alterations in some of the signaling pathways, as well as
cytokines involved in inflammation. It’s an evolving field.
Dr Mani: We’re in the very early stage of understanding
the whole biology of CSCs and developing therapeutics
to inhibit CSCs and their signaling pathways.
Dr Ellis: A CSC is smart enough to know that it will
need multiple pathways to maintain its stemness.
Therefore, combination therapy will be crucial.
SECTION 4. The Next Steps in CSC Research
Identifying CSCs
in multiple tumor types. These include CD133, CD44,
epithelial cell adhesion molecule (EpCAM), and aldehyde
dehydrogenase (ALDH) activity. It must be noted, however,
that no set of markers are exclusive to CSCs, and also that
CSC phenotypes vary over time and between individual
patients’ tumors of the same subtype. These facts
have caused researchers to speculate whether different
To distinguish CSCs from other cancer cells, researchers
have developed profiles of unique cellular markers (Table
2).35,36 These profiles allow detection of CSCs within a
tumor and enable the separation of CSCs from non-stem
cancer cells for research purposes. Several markers have
proven useful for the isolation or enrichment for CSCs
Table 2. T
he CSC Markers Identified in Various Tumor Types35,36
Tumor Type
Phenotype of CSC Markers
Breast cancer
Brain cancer
Colon cancer
ESA+, CD44+, CD24–/low, Lineage–, ALDH1high
CD133+, BCRP1+, A2B5+, SSEA1+
CD133+, CD44+, CD166+, EpCAM+, CD24+, CXCR4+, CK20+, CEA+, LGR5+,
ALDH1high
CD44+, ALDH+, YAP1+, BMI1+
CD34+, CD38–, HLA-DR–, CD71–, CD90–, CD117–, CD123+
CD133+, CD49f+, CD90+
CD133+, ABCG2high
CD138CD133+, CD44+, EpCAM+, CD24+, ABCG2high
CD44+, α2β1high, CD133+
Head and neck cancer
Leukemia
Liver cancer
Lung cancer
Multiple myeloma
Pancreatic
Prostate cancer
Adapted by permission from MacMillan Publishers Ltd on behalf of Cancer Research UK: Chen K, et al. Acta Pharmacologica Sinica. 2013;34:732-740.
8
clinical outcomes reflect different CSC
populations.40 Following is a discussion
of some common markers:
•CD133 is one of the most common
cell surface marker used to identify
CSCs. Originally identified as a
marker for normal stem cells, CD133
is now also known to be present in
CSCs from breast, prostate, pancreas,
brain, and lung cancers. Studies have
also shown that CD133 is useful
not only for detection but also as a
prognostic marker.41
•CD133 has also been shown to detect
peritoneal free cancer cells in patients
with surgically resected colorectal
cancer. In addition, a combination of
CD133 with other markers, CEA and
CK20, was shown to be prognostic for
survival in these patients.42 However,
other studies in Conflict Resolution
Committee dispute the role of some
of these markers in defining CSC
Figure 5. Combination of anti-cancer stem cell (CSC) and conventional
populations.43
treatments. Conventional cancer therapies appear insufficient to kill
all tumor cells. A more rational approach may be to combine
•In human leukemia, a combination
conventional therapies with those aimed at the CSCs and their signaling
of markers CD34, CD38, and IL3Ra
pathways. Such therapies may potentially decrease tumor recurrence
were used to isolate leukemia CSCs.40
and metastases.13,20,38
•Breast CSCs have been identified
using a combination of markers,
ESA+, CD44+, CD24–/low44, and other groups have
Combination Therapy Approaches
shown that elevated ALDH activity in both human and
Multiple research findings indicate that conventional
mouse breast cancer is a marker for CSCs.23
therapies that target the rapidly dividing cells in tumors
have limited efficacy or even adverse effects on CSCs.13
For example, in separate studies of pancreatic cancer
tissues, expression of CD44 in CSCs was shown to
correlate with resistance to chemotherapy and poor
outcomes, whereas elimination of CD133+, tumorigenic,
chemotherapy-resistant CSC populations suppressed
metastasis but did not affect the tumorigenic potential.
Both these findings link CSCs to tumor aggressiveness.13
Combining conventional therapy with CSCdirected therapy may lead to a reduction in overall
tumor mass and a decrease in the risk of relapse and
metastasis (Figure 5).13,20,38 Such a multifaceted treatment
approach might also prevent non-stem tumor cells
from dedifferentiating back to a CSC phenotype, a
phenomenon that has been demonstrated in cell lines.11
Biomarker Tests Under Investigation
Many researchers are investigating biomarkers to
detect CSCs and/or assess their likely response to
different treatments. Some of these findings are
summarized below:
•Gastric CSCs expressing CD44 were resistant to
5-FU, etoposide, and radiation therapy.45
•Colon CSC lines compared with parental cell lines
were resistant to 5-FU and expressed higher levels of
CSC markers CD24, CD44, CD133, and CXCR4.1
•Prostate cancer cells exposed to a clinically relevant
regimen of ionizing radiation, left a resistant
subpopulation that showed increased expression of
stemness markers CD133, SOX2, OCT4, and NANOG.46
9
LOCAL INSIGHTS: Discussing the Evolving Role of Cancer Stem Cells
Two groups of researchers reported an increase in
breast CSC numbers and activity induced by ionizing
radiation therapy. In efforts to counteract this effect, the
groups added additional therapeutic agents directed
toward CSCs. One group added disulfiram plus copper,
which is understood to affect the regulation of stemness
genes via proteasome inhibition,23 and the other added
a γ-secretase inhibitor (to affect the Notch pathway).24
Both combination regimens resulted in a reduction in the
number of radiation-induced CSCs.23,24
These and other data may provide justification for the
development and incorporation of CSC-targeted therapy into
treatment regimens to reduce the likelihood of treatment
resistance, tumor aggressiveness, and tumor regrowth.
They may reduce the tumor burden and ultimately improve
outcomes of patients with CSC-possessing cancers.13,38
CONSULTANT COMMENTARY
Dr Mani: We do have several markers and assays
available to study CSCs. What’s missing is accurate ways
to identify them. One challenge is that many CSC markers
are shared by normal stem cells and are mostly expressed
in particular tissues. For example, a marker used in breast
cancer to identify a CSC may not be useful in a different
tumor. There is an overlap, but it is not necessarily
identical. At the moment, this lack of specific CSC markers
is a major limitation in the identification of CSCs.
Dr Ellis: Another issue is how do you measure CSCs
in breast cancer or colon cancer or pancreatic cancer?
We need more research to help us clearly identify a
CSC population.
Dr Mani: I believe CSC research will change our way of
treating patients. Today, personalized cancer medicine
focuses on the specific mutation in someone’s tumor.
Eventually, personalized treatment will consider CSC
subpopulations as well.
Dr Chang: Chemotherapy alone, combinations of
chemotherapy, or targeted agents alone may not be
the answer. They may not kill all the different clones
of cells with different properties in the tumor, and
they may not kill CSCs. You may need a combination
approach that includes CSC-directed therapies.
CONCLUSION
Although the existence of CSCs was only proven a couple of
decades ago, research suggests they may play fundamental
roles in nearly all cancer processes. It is expected that
oncologists will consider CSCs during the diagnostic workup
and treatment decision-making. Rather than focusing solely
on detecting and eliminating the majority population of
rapidly dividing, terminally differentiated tumor cells,
treatment efforts may also be directed at the subpopulation
of CSCs and related signaling pathways that are the source of
tumorigenesis, resistance, recurrence, and metastasis.6
The identification of signaling pathways that regulate
stemness of CSCs as well as CSC-specific markers that
help isolate CSCs in tumors will provide important
insights into the development of therapies that may
provide greater specificity and decreased toxicity.5,13
A multifaceted therapeutic approach that incorporates
CSC-directed therapies into conventional treatment
regimens may reduce the likelihood of treatment
resistance, tumor aggressiveness, and tumor regrowth
for patents with CSC-possessing cancers.38
10
GLOSSARY
Cancer stem cell (CSC)
CSC markers
Differentiation
Epithelial-mesenchymal
transition (EMT)
Microenvironment
Non-stem cancer cells
Plasticity
Quiescence
Self-renewal
Signaling pathway
Stem cells
Stemness
Treatment resistance
Cancer cell that can self-renew, differentiate into progenitor and non-stem
cancer cells, and generate serially transplantable heterogeneous tumors.
The CSCs may underlie disease relapse and tumor metastasis.3,4,17,47
Protein expression, protein activity, and/or functional cellular signatures used
to identify and enrich CSC populations.3,17
Process by which less specialized cells give rise to more specialized cells,
resulting in a progressive decrease in self-renewal and multilineage
developmental potential.48
A normal tissue organization process that functions by converting
interconnected, anchored epithelial cells to disconnected, motile
mesenchymal-like cells. Pathological activation of EMT in CSCs may support
their stemness and facilitate invasion of lymphatic and circulatory systems to
promote metastasis.35,49,50
Cellular and acellular surroundings of a cell that affect its behavior.51 The
normal stem cell microenvironment, or stem cell niche, maintains stemness.6
Alterations in the normal stem cell niche may produce a pathological niche
supportive of CSC stemness,14 treatment resistance,11 and EMT.25
Non-tumorigenic cells that make up the tumor bulk and have limited
proliferative potential.52
Ability of differentiated cells to generate differentiated cells of the same
or different lineage or to dedifferentiate to more primitive cell states in a
reversible manner. In tumors, non-stem cancer cells may dedifferentiate to
interconvert between differentiated and CSC-like states.3
State of temporary and reversible cellular non-proliferation.53
Defining property of stem cells and CSCs in which parent cells divide to yield
unaltered copies with the same self-renewal and differentiation potential.47,54
Network of interactions between cellular molecules that determine a route
of signals that induce cellular responses.55,56 Inter- or intracellular signaling
molecules bind to receptor proteins, which subsequently alter or activate
target proteins responsible for mediating downstream cellular function.56,57
Dysregulation of signaling pathways underlies CSC maintenance.2
Unique, unspecialized cells capable of long-term self-renewal and
differentiation. 58 However, most stem cells are quiescent,3 and their
differentiated progeny have limited plasticity.3 Stem cells function in tissue/
organ development, injury repair, and tissue regeneration.16,58
Defining characteristics of stem cells, including self-renewal to
preservation of undifferentiated stem cells and production of progenies
that can differentiate.59
An underlying cause of cancer progression and recurrence. Diverse mechanisms,
such as anti-apoptosis protein overexpression, drug efflux proteins, DNA
repair activation, low reactive oxygen species levels, quiescence, aberrant
developmental signaling, and microenvironmental stimuli, contribute to the
treatment-resistance property of CSCs.60
11
LOCAL INSIGHTS: Discussing the Evolving Role of Cancer Stem Cells
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