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can self-renew indefinitely. Many pathways vital to SC the combination of CD44 and CD24 markers. [2]
function, such as Wnt, Hedgehog, Notch, and PI3K/Akt,
[7]
[8]
are dysregulated in CSCs, potentially contributing to Functional CSC properties like intracellular ALDH
neoplastic transformation. For example, cases of multiple enzymatic activities and ABC transporter efflux activities
myeloma have displayed abnormal signaling in response of vital DNA dyes such as Hoechst 33342 have been used
to elevated levels of Hedgehog ligand secreted by tumor for CSC isolation. [21,22] Increased aldehyde dehydrogenase
stromal cells, and upregulated Notch4 signaling has been isoform 1 (ALDH1) activity has been used to identify
[9]
implicated in drug-resistant breast CSC activity. Like and analyze different types of CSCs. Furthermore, CSCs
[10]
SCs, CSCs are able to repair damaged DNA more quickly have a distinct efflux mechanism, stemming from their
[15]
and overexpress drug-efflux pumps such as ATP-binding high expression of ABC transporter proteins. These
cassette (ABC) transporters. In a glioblastoma model, cells, referred to as the “side population” (SP), are able to
aberrant Akt signaling contributed to overactivation of the actively transport fluorescent dyes such as Hoechst 33342
ABC transporter ABCG2 in CSCs, leading to increased drug out of the cells. Flow cytometric SP analysis has been
expulsion and rendering them resistant to mitoxantrone. [11] performed with numerous cancer cell lines and the SP has
shown enriched CSC activities. [21]
CSCs may also contribute to metastasis. During normal
wound healing, cells are able to migrate to the wound site A subpopulation of CSCs exhibit intrinsic autofluorescence
through the epithelial to mesenchymal transition (EMT) and were shown to be exclusively linked to a functional
process. CSCs may also undergo EMT when migrating CSC phenotype in different epithelial tumors. These
from the primary tumor site. Another theory hypothesizes autofluorescent cells had CSC characteristics such as
that the CSC microenvironment -- including the high self-renewal, long-term tumorigenic capacity,
surrounding vasculature -- facilitates metastasis. While invasiveness, and chemoresistance. These cells have
[12]
the exact mechanisms have not been discovered, there are intrinsic autofluorescence with excitation wavelengths at
many reports of CSC-driven metastasis. In fact, numerous 488 nm and emission at about 520 nm. This new marker
studies have used breast CSC-rich cell lines such as MDA- has been proven to be a more reliable and accurate way to
MB-231 to first produce primary tumors and then seed identify and characterize CSCs. [16,23]
lung metastases. [13,14]
Another important functional property of CSCs, as well as
Studies of CSC-targeted therapy depend on the isolation normal stem SCs, is the ability to produce sphere-forming
and enrichment of CSCs. They can be identified, isolated, colonies from a single cell in serum-free medium or in soft
and characterized by several methodologies, including flow agar medium, as differentiated cells cannot survive and
cytometric analysis of CSC-specific cell surface markers, proliferate in this environment. Thus, several studies
[24]
detection of side-population (SP) phenotypes by Hoechst have used the sphere formation assay as an efficient
33342 dye exclusion, changes in aldehyde dehydrogenase method for isolating, enriching and maintaining CSCs
(ALDH) enzymatic activities using an aldeflour assay, from various primary tumors. Generally, these CSC-driven
ability to grow as suspension spheres in serum-free medium, spheres are greater in both number and size as compared
SC-related gene expression, and auto-fluorescence. [6,15-17] to ones generated from non-CSCs. [18,25] These spheres
There are no widely accepted techniques solely developed clearly demonstrated stem-like properties and expressed
to isolate CSCs, necessitating the use of combination characteristics of CSCs. [16]
markers and methods rather than single strategies.
Here, we will focus on cancer therapeutics which can
Surface marker-based assays have become the mostly target CSCs. The development of various strategies that
commonly used method. Table 1 summarizes the list can act effectively against CSCs has been categorized into
[18]
of cell surface phenotypes of CSCs in different tumors. six groups, as shown in Figure 1.
The detection can be performed with specific antibodies in
flow cytometry, competitive ELISA, or magnetic beads. REGULATING CSC SIGNALING
[19]
Dick and coworkers showed the first evidence of the PATHWAYS
presence of CSCs in human AML by the flow cytometric
display of the CD34 CD38 surface marker phenotype. Many signaling pathways are deregulated in CSCs and
-
+
[20]
A breast CSC subpopulation was identified and isolated by are potential targets in anti-CSC therapies. Overactivation
Table 1: Cancer stem cell surface markers in human cancers
Tumor types Surface marker on the CSCs References
Breast CD44+/CD24-, CD133+, EpCAM+ [2,17,18]
Colon CD133+, EpCAM+, CD44+ [17,18,36]
Glioma (brain) CD133+, CD15+, CD49f+, CD90+ [3,17,18]
Leukemia (AML) CD34+/CD38-, CD123+ [1,17,18]
Lung ABCG2, CD133+ [16-18]
Melanoma ABCB5, CD133+, CD20+, CD271+ [18]
Ovarian CD44+, CD117+, CD133+ [39]
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Journal of Cancer Metastasis and Treatment ¦ Volume 2 ¦ July 8, 2016 ¦