Cancer Stem Cells and Cancer Stem Cell Inhibitors in Gastrointestinal Cancers

Oncology & Hematology Review, 2016;12(1):41–6 DOI: http://doi.org/10.17925/OHR.2016.12.01.41

Abstract:

Cancer stem cells (CSCs) are a subpopulation of phenotypically distinct cancer cells that may play an important role in tumor pathogenesis. The gastrointestinal (GI) system provides a good example for investigation of the role of CSCs in tumor proliferation; GI CSCs are suitable for study due to their abundance, proliferative potential, and consistent structural arrangement that is maintained under tightly controlled signaling pathways. GI stem cells have a long lifespan and this, combined with their rapid turnover, may predispose them to forming CSCs. Alternative possible sources of GI CSCs include differentiated intestinal cells, bone marrow, and cancer cells. Therapies that specifically target CSCs present an exciting opportunity to treat patients with cancer. Enhanced understanding of CSC markers, such as CD133, CD44, and epithelial cell adhesion molecule (EpCAM), may facilitate development of therapies that target them. Among the stemness pathways that have been targeted are Wnt/β-catenin, STAT, Notch, and Nanog.
Keywords: Cancer therapy, cancer stem cells, CD133, CD44, epithelial cell adhesion molecule (EpCAM), focal adhesion kinase (FAK), gastrointestinal tumor, Hedgehog, Nanog, Notch, STAT3, Wnt/β-catenin
Disclosure: Joleen M Hubbard receives research support from Boston Biomedical. Axel Grothey has nothing to disclose in relation to this article.
Acknowledgments: Medical writing assistance was provided by Catherine Amey at Touch Medical Media, funded by Boston Biomedical.
Open Access: This article is published under the Creative Commons Attribution Noncommercial License, which permits any noncommercial use, distribution, adaptation, and reproduction provided the original author(s) and source are given appropriate credit.
Received: January 31, 2016 Accepted March 24, 2016
Correspondence: Axel Grothey, Division of Medical Oncology, Mayo Clinic, 200 First St SW, Rochester, MN55905, US. E: grothey.axel@mayo.edu
Support: The publication of this article was supported by Boston Biomedical, who were given the opportunity to review the article for scientific accuracy before submission. Any resulting changes were made at the author’s/authors’ discretion. The publication of this article was supported by Boston Biomedical. The views and opinions expressed are those of the author and do not necessarily reflect those of Boston Biomedical.

As is the case with most solid tumors, gastrointestinal (GI) tumors are treated in a variety of modalities, which are used singularly or in combination, including surgery, targeted therapies, radiation, and chemotherapy. Survival rates by stage in colon and rectal cancer are given inTable 1.

Cancer stem cells (CSCs) are believed to be malignant cells that have the capacity to initiate and maintain tumor growth and survival.1 They are more resistant to radiation and chemotherapeutic agents than other cancer cells.2 The presence of CSCs following cancer therapy may explain the initiation of metastasis and later recurrence of cancers, which can occur even when there is a good initial response to radiation or chemotherapy.3 The first experiments suggesting the presence of CSCs in GI cancers were conducted in 2007.4,5 The investigators used flow cytometry to isolate CSCs using CD133 as a marker and then demonstrated the ability of these CD133-positive cells to form xenografts in non-obese diabetic/severe combined immunodeficiency mice.

Origin of cancer stem cells of the gastrointestinal system
A schematic representation of an individual colon crypt is depicted in Figure 1. Stem cells lie at the bottom of the crypt and through asymmetric division are responsible for generating all epithelial cell types along the crypt–villus axis. GI stem cells may be intrinsically prone to forming CSCs because of their long lifespan combined with rapid turnover and it is widely believed that CSCs are derived from normal stem cells.6,7Other possible sources for GI CSCs include dedifferentiated intestinal cells, possibly via nuclear factor-kappa-B (NF-κB) modulation of Wnt signaling7,8 and bone marrow-derived progenitor cells progressing through metaplasia and dysplasia to cancer.9

Cancer stem cell biomarkers
The CD133 molecule, which is also known as prominin-1, is a pentaspantransmembrane glycoprotein that has been shown to be located mainly in membrane protrusions.10 It was first identified as a surface protein marker of a subset of hematopoietic stem and progenitor cells as early as 1997,11 but its biologic function has yet to be elucidated. In 2007, CD133-positive cells separated from colorectal tumor cells were demonstrated to possess self-renewal properties and high tumorigenic potential.4,5Systematic reviews have indicated that CD133 is a prognostic factor in colorectal cancer (CRC)12 and gastric cancer.13 Carbon nanotube-conjugated CD133- positive monoclonal antibodies led to photothemolysis of CD133-positive glioblastoma cells in vivo and in mice,14suggesting that CD133 may represent a useful target to selectively inhibit CSCs in CD133-expressing tumor types.

CD44
CD44, a cell surface adhesion molecule, is the principal receptor for hyaluronate, which is the most abundant extracellular matrix component.15,16 CD44 has been suggested to perform functions in CSCs such as mediation of adhesion and homing to the stem cell niche, enhancement of antiapoptotic proteins and surface efflux pump expression, regulation of the cellular redox status and the response to the activation of the canonical Wnt pathway.17–19 Future research is needed to elucidate the suitability of CD44 as a CSC marker in GI cancer and its role in tumorigenesis.

EpCAM
Epithelial cell adhesion molecule (EpCAM), initially described in human CRC as a tumor-associated antigen,20 is expressed highly in a range of human epithelial normal and cancer tissues, including the colon.21 Several lines of evidence indicate that EpCAM is involved in cell adhesion, proliferation, migration, and cancer and stem cell signaling.22,23

Other potential markers
More recently, identified possible markers of CSCs include: CD29/integrin β1, a mucin-like cell adhesion molecule;24 CD24/HSA, a extracellular matrix protein receptor that is involved in regulation of cell migration, proliferation, survival, differentiation, and death;25 Lgr5/Gpr49, a receptor for R-spondin proteins;26and CD166/ALCAM, a cell adhesion molecule.27

Limitations of cancer stem cell markers
Identification and isolation of CSCs using putative surface markers has received much attention in cancer research. However, heterogeneity among GI tumors and GI tumor subtypes has led to difficulty in pinpointing unique markers. Expression of surface markers varies at different tumor stages and their main regulatory functions are not understood fully.28 Lack of universal expression of surface markers has obfuscated their use and no optimal combination of markers has been confirmed for the identification of CSCs. Further, non-CSCs have been shown to also express some of these markers.29 CSC markers are an area of continuous development as more studies identify molecules that may serve as new CSC markers and help to identify CSCs in GI cancers in a tissue-specific manner.

Cancer stemness and epithelial-to-mesenchymal transition
Stemness, initially taken to mean expression of stem cell genes, such as Nanog, Oct4, and Sox2, is a defining property of embryonic and adult stem cells.30Stemness can be measured by a cell’s ability to form spheres when cultured in stem cell media.31 Chemotherapy32 and radiation33 have been found to induce the expression of stemness genes in cancer cells in vitro, thereby enriching the CSC population in the residual tumor.

Epithelial-to-mesenchymal transition (EMT) is the capacity of epithelial cells to acquire mesenchymal traits to allow local invasion into surrounding tissues and systemic dissemination to distant organ sites.3 Recent evidence indicates that EMT can induce differentiated cancer cells into a CSC-like state, suggesting a functional link between stemness and EMT.34

Stemness pathways associated with cancer stem cells in gastrointestinal malignancies— therapeutic targets
The desired aim is to overcome resistance to chemotherapeutics and reduce therapy-related toxicity by developing treatments that are specific for CSCs and that are not toxic to healthy tissues. Examples of stemness pathways as therapeutic targets are discussed below.

Nanog
Nanog, an essential regulator of embryonic stem cell self-renewal that inhibits differentiation, is overexpressed in a variety of cancers including those of the GI system35,36 High levels of Nanog expression are associated with advanced stages of cancer and a poor prognosis, suggesting that it may play a key role in tumor transformation, tumorigenesis, and tumor metastasis.35 Nanog is involved in a complex regulatory network that determines cell fate, proliferation, and apoptosis (Table 2). It is therefore a promising therapeutic target. Genetic ablation of Nanog in SW620 colorectal carcinoma cells suppressed both tumor growth in athymic nude mice and cell proliferation in vitro.37Further, ablation mediated by short hairpin RNA decreased the expression of core CSC transcription factors, supporting a role for Nanog as a signali ng hub in CSCs.38

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Keywords: Cancer therapy, cancer stem cells, CD133, CD44, epithelial cell adhesion molecule (EpCAM), focal adhesion kinase (FAK), gastrointestinal tumor, Hedgehog, Nanog, Notch, STAT3, Wnt/β-catenin