Introduction
Cancer stem cells
Properties of cancer stem cells
-
Self-renewal refers to the ability to form new stem cells with an identical and intact potential for proliferation, expansion, and differentiation, thus maintaining the stem cell pool. Self-renewal mechanisms that allow stem cells to persist consistently involve proto-oncogenic pathways, such as the Wnt/β-catenin and Notch pathways. Another regulator of self-renewal in the context of embryogenesis is the sonic hedgehog (Hh) signaling pathway (reported in multiple myeloma); however, little is known about the role of this pathway in adult stem cells and CSCs [21]. The preferential expression of Hh in CSCs was first published in a pancreatic cancer xenograft model [22], and evidence that the Hh pathway is aberrantly activated in a number of solid tumors, including colon cancer, has also been published [23].A variety of signals have been shown to promote the self-renewal capacities of colon CSCs, including the Wnt pathway and the prevention of β-catenin-dependent transcription. In addition, DLL4 stimulates Notch receptors on neighboring cells and, together with β-catenin, directs an immature transcription profile that promotes self-renewal. BMP4 is also known to counteract this self-renewal activity of CSCs by binding to BMP receptors, thereby interfering with Wnt signaling and subsequently promoting differentiation. Last, hepatocyte growth factor (HGF) has been shown to maintain colon CSCs in a stem-cell state and prevent differentiation [24].Homeostasis (i.e., CSC maintenance and proliferation) of the intestinal epithelium is tightly controlled and depends on the spatial organization of signals that emanate from supportive mesenchymal cells, the stromal environment, and differentiated epithelial progeny, although it remains unclear how these latter cells are integrated into the organization of intestinal cancers [25].Increased numbers of CSCs may occur in poorly differentiated tumors (through asymmetric cell division and damaged stem cells) as well as advanced tumors where the tumor microenvironment promotes EMT, resulting in CSC expansion. Furthermore, activation of these pathways in stem cells over the life span of an organism may predispose these cells to neoplastic transformation and homeostatic proliferation.
-
Differentiation is defined as the ability to develop into a heterogeneous progeny of cells, which progressively diversifies and specializes according to a hierarchical process, constantly replenishing the tissue of short-lived, mature elements [26]. Recent reports about colon cancer have suggested that individual tumors, at the histopathological level, are relatively undifferentiated and may contain higher proportions of CSCs than their more differentiated counterparts, which have a significantly worse clinical prognosis [27,28].
-
Homeostatic control is the ability to modulate and balance differentiation and self-renewal [26]. Recently, it was shown that differentiated cells in the intestinal epithelium reside in the intestinal crypts as at least two types of stem cells leucine-rich repeat containing G protein-coupled receptor 5 (Lgr5) and B lymphoma Mo-MLV insertion region 1 homolog (Bmi-1), which serve to maintain the regenerative capacities of this tissue under homeostatic conditions [29]. Lgr5 expressing cells are the more active stem cell type and serve to maintain the regenerative capacities of these tissues under homeostatic conditions. In addition, Lgr5 expressing cells are actively proliferating and extremely sensitive to Rspo1-mediated Wnt stimulation and Dkk1-mediated Wnt inhibition. In contrast, Bmi-1-expressing cells are less affected by environmental stress (i.e., not sensitive to Wnt modulation), normally quiescent and are held in reserve for “special occasions”, in which they give rise to progeny that clonally repopulate multiple contiguous crypt-villus axes during subsequent intestinal regeneration [30]. However, the homeostasis of tumoral epithelial tissues is governed by a complex program, which is controlled by niche-dependent signals that involve the subepithelial stroma (VEGF, platelet-derived growth factor, TGF-β, Nuclear factor kB), adjacent epithelial cells (Notch, Hedgedog), natural enteric flora, as well as intracellular transcription factors and the activation of signaling networks associated with the epithelium (i.e., Wnt-β-catenin). As previously reported, Wnt proteins and the Notch pathway are crucial for maintaining stem cell homeostasis, as these signals have the potential to maintain the phenotype of CSCs in the tumor mass [31]. However, the equilibrium that regulates the growth and maintenance of tumors is poorly understood.
CSC markers
Malignant transformation of colon cancer
Model of colon carcinogenesis
Cancer stem cells and carcinogenesis
Chemoresistance
Cancer stem cells and chemoresistance
Signals of chemoresistance
Perspectives on treatment
Target
|
Compound
|
Class
|
Status
|
Reference
|
---|---|---|---|---|
anti-EpCAM/anti-CD3
| MT110 (solitomab) | BITE; human recombinant single chain bispecific bifunctional mAb construct | Preclinical, in vitro, xenograft mice Phase I clinical study, advanced solid tumors | [82] |
anti-EpCAM/anti-CD3/Fcγ
| Catumaxomab (Removab™, TRION Pharma, Germany) | Triomab; recombinant chimeric two half antibody, each with one light and one heavy chain from mouse IgG2a and rat IgG2b isotypes. Bispecific, trifunctional mAb construct | Phase I–III clinical studies | [85] |
anti-IGF-IR
| AVE1642 | Humanized recombinant IgG1 mAb, derived from mouse anti-IGF-IR IgG1mAb EM164 | Preclinical, xenograft mice | [78] |
Figitumumab (CP-751,871) | Humanized IgG2 mAb | Preclinical, in vitro, xenograft mice | [81] | |
anti-DLL4
| OMP-21 M18 (Demcizumab) | Humanized IgG2 mAb | Preclinical, xenograft mice Phase I clinical studies, combination with drugs | |
anti-Frizzled (1, 2, 5, 7, 8)
| OMP-18R5 (vanticumab) | Humanized recombinant IgG2 mAb | Preclinical, xenograft mice | |
Drug efflux protein multidrug resistance 1 (MDR1)
| Lipid nanocarriers | PEI-lipid nano complex with an MDR1-targeting siRNA (siMDR1) | Human colon CSC (CD133+ enriched cell population) | [84] |
Cancer stem-like cells (CSLCs) that are resistant to conventional chemotherapy and the bulk cancer cells
| CSO-SA/OXA micelles | Micelle formulation of oxaliplatin (OXA) encapsulated in chitosan vesicle |
In vitro (HT29 and SW620 line cellular [CD133+/CD24+]) | [87] |