Introduction and Origin of Cancer Stem Cells
Cancer stem cells (CSCs) can be defined as cells in the tumor growth with a tumor initiating potential. Normal stem cells are characterized by three properties: 1 Capability of self-renewal; 2 Strict control on stem cell numbers; 3 Ability to divide and differentiate to generate all functional elements of that particular tissue. Compared to normal stem cells, the cancer stem cells are believed to have no control on the cell numbers. Cancer stem cells form very small numbers in whole tumor growth and they are said to be responsible for the growth of the tumor cells. The major difference between NSCs and CSCs lies in the ability to regulate stemness pathways. In CSCs, deregulation of the stemness pathways (including Wnt/β-catenin, Janus kinase/signal transducer and activator of transcription [JAK/STAT], transforming growth factor-beta [TGF-β], Notch, Hippo, etc) along with improper interactions between them may represent key events for CSC propagation and pathogenesis.
Cancer stem cells are derived from either the self-renewing normal stem cells or from the progenitor cells that have acquired the ability of self-renewal due to mutations (figure 1).
Figure1. A simplified model of suggested hypothesis about origin of the cancer stem cell
The hypothesis that cancer stem cells are derived from normal stem cells rather than more committed progenitor cells have been addressed in the cases of AML where leukaemia initiating cells (LIC) from various subtypes of AML with different stages of differentiation have been shown to share the same cell-surface markers with normal haematopoietic stem cells. In the solid tumors, there have been identification of cell-surface markers in the lung, brain and prostate which may allow the separation of the stem or progenitor cells with the tumor initiating function.
Functional Properties and Microenvironment of CSCs
CSCs can undergo symmetric cell division, which leads to the production of two identical daughter CSCs. They can also undergo asymmetric cell division to produce one daughter CSC and a daughter progenitor cell capable of expanding into different types of tumor tissue. CSCs use asymmetric cell division to expand the tumor population while maintaining CSCs.
The microenvironment of CSC, also named niche is composed of CSCs’ surroundings (niche cells, stromal cells, immune cells, and vasculature) and secreted factors derived from these cells, which provide a “fertile soil” for the CSC to propagate the tumor cells. The CSCs’ niche provides a regulatory microenvironment for CSCs that exhibit stem cell-like characteristics and have the ability to regenerate the bulk of tumor cells while maintaining self-renewal potential. Given the complexity of stem cells, there are both internal and external signals in place to ensure a balance between stem cells and their progeny. The internal signals include molecular pathways designed to guide differentiation. The external signals are in part made up of a micro-environment or niche composed of cells designed to anchor the stem cells to the specified environment and secrete factors required for maintaining stem cells in a quiescent and undifferentiated state. The release of stem cells from a specified niche may lead to differentiation of the stem cell.
Identification of CSCs
Investigation of CSCs in multiple tumor types have identified markers specific to these cells. (A summary of CSC markers is presented in Table1.)
Table 1. CSC Markers
|Tumor type||Phenotype of CSC markers|
|Colon cancer||CD133+, CD44+, CD166+, EpCAM+, CD24+, CXCR4+, CK20+, CEA+, LGR5+|
|Head and neck cancer||CD44+, ALDH+, YAP1+, BMI1+|
|Leukemia||CD34+, CD38-, HLA-DR-, CD71- CD90-, CD117-, CD123+|
|Breast cancer||ESA+, CD44+, CD24–/low, Lineage–, ALDH1high|
|Liver cancer||CD133+, CD49f+, CD90+|
|Brain cancer||CD133+, BCRP1+, A2B5+, SSEA1+|
|Lung cancer||CD133+, ABCG2high|
|Prostate cancer||CD44+, α2β1high, CD133+|
|Pancreatic||CD133+, CD44+, EpCAM+, CD24+, ABCG2high|
CD133 is one of the most common cell surface markers used to identify CSCs.CD133 was originally identified and cloned by the Miraglia group as a hematopoietic stem and progenitor cell marker, which was enriched in CD34+ leukemia cells(leukemia stem cells) CD133 is also present in CSCs from many malignancies, including breast, prostate, pancreas, and lung.
Aldehydedehydrogenase1 (ALDH1) enzymatic activity is considered an internal marker for normal and malignant stem cells and progenitor cells. High ALDH1 levels correlated with therapy resistance, aggressive phenotype, and worse clinical outcome in multiple cancers, including esophageal and breast. The major stemness transcriptional factors, Nanog, OCT4,and SOX2,are also overexpressed in CSCs. Modulation of Nanog directly affects stemness of colon CSCs, as well as colorectal cancer proliferation, tumorigenicity, and metastasis. In addition, increased expression of OCT4 and Nanog has been shown to increase lung CSC count and tumor-initiating capabilities, increase resistance to chemotherapy, and promote EMT. Recent evidence suggests YAP1 is an important intracellular CSC marker and endows stemness properties on to a wide variety of non-transformed cell types of gastrointestinal origin, including primary esophageal epithelium cells, immortalized embryonic liver cells, and esophageal cancer cells, through regulation SOX9. Multiple publications have shown that the polycomb protein BMI1 (B cell-specific Moloney murine leukemia virus integration site 1) can serve as an intracellular CSC prognostic marker in multiple head and neck cancers.
The signaling pathways of cancer stem cells allow a more precise targeting mechanism for the eradication of cancer cells responsible for tumor progression. The following are several pathways which have been studied already.
The canonical Wnt pathway is a critical component for the maintenance of homeostasis in the highly proliferative intestinal crypt and epithelium. In many mammalian cells, Wnt ligands, which are produced from cells in the stem cell microenvironment, serve as a self-renewal signal for the stem cells.
In embryonic stem cells, STAT3 forms a complex with Nanog, which translocates to the nucleus to transcribe genes required for maintaining pluripotency. A similar binding complex is present in head and neck cancer to enhance oncogenesis and chemoresistance via CD44-mediated expression of Micro RNA-21 in head and neck cancer. STATs are activated upstream by the JAK family. In prostate cancer, blockade of the STAT3 signaling has been shown to inhibit the clonogenic and tumorigenic potential of CSCs. Colon CSCs (ALDH+/CD133+) exhibit a higher level of STAT3 phosphorylation compared with ALDH-/CD133- or nonseparated cells. In addition, blockade of STAT3 activity has been shown to inhibit the tumor growth and tumor-initiating potential in these cells.
The Hedgehog (HH) pathway is essential for maintaining the complex types of replication and generation of specialized cells during development. The HH pathway continues to function in stem and progenitor cells from embryonic development to maintenance of fully grown adult organs, including the brain, skin, prostate, and bladder. HH signaling activates the expression of many genes involved in cell proliferation, differentiation, invasion, and survival.
The Notch pathway contributes to processes that determine numerous aspects of cellular functions, including embryogenesis and angiogenesis. Activation of Notch leads to the sequential cleavage of the Notch receptor by TACE (tumor necrosis factor-α-converting enzyme)and γ-secretase into a mobile product that translocates to the nucleus to activate transcription of targets, including Hes1, Hey1, and cyclin D. Notch pathways have been associated with the propagation of CSCs and induction of EMT, leading to drug resistance.
TGF-β signaling has been shown to induce cellular growth, arrest, differentiation, and apoptosis depending on the combination of TGF-β cytokines, receptors, and effector SMADs activated. In normal and premalignant cells, TGF-β signaling enforces homeostasis and suppresses tumor progression.
Evidence for Cancer Stem Cell Involvement in Specific Malignancies
The colon is comprised of highly proliferative cells that are well organized under tightly controlled signaling pathways. Among these pathways, the Wnt/β-catenin pathway has a role in maintaining epithelial crypt homeostasis. Barker and colleagues showed that loss of adenomatous polyposis coli (APC), an inhibitor component of the Wnt/β-catenin pathway, induced adenoma formation in the pylorus. In most colorectal cancers, loss of APC is one of the hallmark events of transformation of normal cells into cancerous cells. APC also down-regulates the expression of Survivin protein in the normal intestinal crypt, thereby increasing epithelial apoptosis that limits the population of intestinal stem cells as well as other epithelial cells in the crypt. APC inactivation and mutation is very common in colorectal cancers, possibly as a result of Survivin overexpression.
The role for CSCs in tumorigenesis was first identified by Bonnet and Dick in the late1990s. Scientists isolated a CD34+ subpopulation of acute myeloid leukemia cells that initiated AML in nonobese diabetic mice with severe combined immunodeficiency. These cells displayed characteristics similar to normal HSCs. Later, Hope and Dick provided direct evidence for self-renewal of individual leukemic CSCs and found that this stem cell population was not functionally homogenous. Instead, like normal HSCs, this population was comprised of a distinct hierarchy of leukemic stem cell classes (short-term, long-term, and quiescent) with heterogeneous self-renewal capacity. More recently, scientists shown the clinical relevance of CSCs by identifying a shared core gene signature between leukemia stems cells and HSCs controlling the “stemness” in both groups, especially in CD34+/CD38- stem cells.
Evidence for the role of CSCs in initiating and perpetuating breast cancer has been available since at least 2003, when Al-Hajj and colleagues isolated a tumorigenic subset of CD44+ CD24-/low lineage (CD2, CD3, CD10, CD16, CD18, CD31, CD64, CD140b) cells, that when as few as 100 cells were injected into mice, formed tumors. Subsequent studies have found that tumors with this phenotype have a generally poor prognosis.
The Potential of CSCs as A Target for Cancer Treatment
Several pathways above on which CSCs depend have been identified and will represent future therapeutic targets. Clinical trials are under way evaluating agents that target stemness pathways, including Wnt, TGF-β, Notch, Hedgehog, and JAK/STAT pathways, in a wide variety of cancers, including brain, breast, colon, gastric, lung, pancreas, prostate, and hematologic malignancies. Several studies are evaluating agents targeting these pathways, including some in combination with conventional chemotherapies. The hope is that targeting the root and stem causes of cancer will result in improvement of long-term outcomes.