**3.4. Origin of the gastric cancer stem cells (CSCs)**

The origin of gastric cancer stem cells (CSCs) is described as follows:


Helicobacter pylori infection triggers inflammation and changes the local gastric microenvironment. This change might affect the differentiation of gastric stem cells and could induce gastric cancer. Helicobacter pylori colonizes and manipulates both progenitor and leucinerich repeat containing G protein-coupled receptor-5 (Lgr5+ ) stem cells, which then change gland turnover and cause hyperplasia [28].

Gastric Cancer with Liver Metastasis (GCLM) and the Importance of Dormant Cancer Stem Cells http://dx.doi.org/10.5772/intechopen.69829 249

**Figure 2.** Gastric cancer stem cells formation [28].

**3.3. Brief history of cancer stem cells (CSCs)**

**Table 1.** Characteristics of the cancer stem cells [21].

248 Gastric Cancer

form during organogenesis were "lost."

**3.4. Origin of the gastric cancer stem cells (CSCs)**

The origin of gastric cancer stem cells (CSCs) is described as follows:

**1.** CSCs are derived from progenitor and normal stem cells [27].

rich repeat containing G protein-coupled receptor-5 (Lgr5+

gland turnover and cause hyperplasia [28].

surface marker CD34 (CD34+

and brain tumors [26].

aling [27].

cancer (**Figure 2**) [29].

History of cancer stem cells dates back to the nineteenth century. A hypothesis of cancer stem cells (CSCs) that have similar properties to stem cells (SCs) was first described by Rudolf Virchow and Julius Conheim in 1855 [23]. Virchow suggested that cancers arise from the activation of dormant cells present in mature tissue, which are remainders of embryonic cells (perhaps similar to cells now known as stem cells) [23]. Virchow believed that cancer is caused by severe irritation in the tissues, and his theory came to be known as chronic irritation theory. However, Conheim had suspected that the remaining embryonic cells from which cancers

Self-renewal CSCs serially transplant through multiple generations Differentiation CSCs generate symmetrical and asymmetrical cells

Specific surface markers Allow for separation of CSCs from nonstem cells

Tumorigenicity CSCs can propagate tumor cells

In 1997, Bonnet and Dick described a subpopulation of cells with the presence of a specific

acute myeloid leukemia capable of inducing a cancerous disease after transplanting those

CSCs have already been identified in breast, lung, ovarian, prostate, gastric, colorectal cancer,

**2.** Dedifferentiated gastric cells, via nuclear factor-kappa-B (NF-κB) modulation of Wnt sign-

**3.** Bone marrow-derived progenitor cells progressing through metaplasia and dysplasia to

Helicobacter pylori infection triggers inflammation and changes the local gastric microenvironment. This change might affect the differentiation of gastric stem cells and could induce gastric cancer. Helicobacter pylori colonizes and manipulates both progenitor and leucine-

cells to mice with an altered immunological system—leukemic-initiating cells [24, 25].

It is estimated that in these malignancies, CSCs constitute <5% of all tumor cells [26].

) and the absence of a CD38 marker (CD38<sup>−</sup>

) in patients with

) stem cells, which then change

Chronic infection with Helicobacter felis caused inflammation and induced the reconstruction of gastric tissue with bone marrow-derived cells, whereas acute inflammation does not lead to bone marrow-derived cell recruitment [29].

### **3.5. Cancer stem cells (CSCs) 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 [30]. 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 [31–33]. 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 (**Table 2**) [32].

Using glioma stem cells, research has shown that CSCs can divide symmetrically, producing new CSC progeny, or asymmetrically, producing nonstem cell and stem cell progeny [34]. Intratumoral heterogeneity likely derives from asymmetrical division and differentiation of CSCs [33]. Over time, unrestrained differentiation and proliferation produces the heterogeneous


**Table 2.** Characteristics of normal stem cells and cancer stem cells [32].

populations of primary and metastatic tumor cells that contribute to tumor properties, such as recurrence, resistance to therapy, and metastasis [30].

The manifestation of CSCs heterogeneity:

First, different subsets of cancer stem cells express different surface markers. Wright et al. described that breast CSCs could be divided into CD44+ /CD24<sup>−</sup> and CD133+ subsets based on differences in surface marker expression [35].

Second, the heterogeneity of CSCs is manifested in the differences of the cell properties. Specifically, some cancer stem cell subsets possess a strong invasive capability, whereas other cancer stem cell subsets are in a quiescent (dormant) state and do not differentiate [36, 37].

Third, the dormant state of cancer stem cells is not permanent. Under the influence of appropriate external or internal stimuli, dormant cancer stem cells may undergo invasive transformation and become invasive cancer stem cells [38]. Therefore, an investigation of the factors that promote quiescent stem cell transformation is of great clinical significance.

### **3.6. Cancer stem cells (CSCs) dormancy**

Many solid tumors undergo an extended period of "dormancy," characterized by the presence of minimal residual disease over many years before overt metastases may eventually arise.

Gastric CSCs consisted of both quiescent gastric CSCs and invasive gastric CSCs (increased metastatic activity). Invasive gastric CSCs are defined as CD26<sup>+</sup> CXCR4+ double-positive cells and the CD26<sup>−</sup> CXCR4<sup>−</sup> double-negative cells as quiescent gastric CSCs based on surface marker expression [39].

In 2007, Aquirre-Ghiso postulated two different states of "cancer dormancy," tumor-cell dormancy, and tumor mass dormancy [19, 40, 41].

Tumor mass dormancy (micrometastasis) occurs when cancer cell proliferation is counterbalanced by apoptosis owing to poor vascularization (angiogenic dormancy) or by an immune response. In this case, the cancer cells are never truly inactive, but rather are incapable of expanding beyond a certain number (**Figure 3**).

Tumor-cell dormancy is defined as the condition in which cancer cells enter the G0 phase of the cell cycle and have low metabolism. This form of dormancy is clinically asymptomatic.

However, this conceptual framework is still under debate. At present, little is known about the factors that might have a role in the "awakening" of dormant tumor cells that leads them into the dynamic phase of macrometastatic formation.

CSCs exist within a microenvironment of surrounding vasculature, stromal cells, immune cells, and secreted factors produced by these cells. 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 Gastric Cancer with Liver Metastasis (GCLM) and the Importance of Dormant Cancer Stem Cells http://dx.doi.org/10.5772/intechopen.69829 251

populations of primary and metastatic tumor cells that contribute to tumor properties, such as

First, different subsets of cancer stem cells express different surface markers. Wright et al.

Second, the heterogeneity of CSCs is manifested in the differences of the cell properties. Specifically, some cancer stem cell subsets possess a strong invasive capability, whereas other cancer stem cell subsets are in a quiescent (dormant) state and do not differentiate

Third, the dormant state of cancer stem cells is not permanent. Under the influence of appropriate external or internal stimuli, dormant cancer stem cells may undergo invasive transformation and become invasive cancer stem cells [38]. Therefore, an investigation of the factors

Many solid tumors undergo an extended period of "dormancy," characterized by the presence of minimal residual disease over many years before overt metastases may eventually

Gastric CSCs consisted of both quiescent gastric CSCs and invasive gastric CSCs (increased

In 2007, Aquirre-Ghiso postulated two different states of "cancer dormancy," tumor-cell dor-

Tumor mass dormancy (micrometastasis) occurs when cancer cell proliferation is counterbalanced by apoptosis owing to poor vascularization (angiogenic dormancy) or by an immune response. In this case, the cancer cells are never truly inactive, but rather are incapable of

Tumor-cell dormancy is defined as the condition in which cancer cells enter the G0 phase of the cell cycle and have low metabolism. This form of dormancy is clinically asymptomatic.

However, this conceptual framework is still under debate. At present, little is known about the factors that might have a role in the "awakening" of dormant tumor cells that leads them

CSCs exist within a microenvironment of surrounding vasculature, stromal cells, immune cells, and secreted factors produced by these cells. 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

that promote quiescent stem cell transformation is of great clinical significance.

metastatic activity). Invasive gastric CSCs are defined as CD26<sup>+</sup>

/CD24<sup>−</sup>

and CD133+

CXCR4+

double-negative cells as quiescent gastric CSCs based on surface

subsets based on

double-positive cells

recurrence, resistance to therapy, and metastasis [30].

described that breast CSCs could be divided into CD44+

The manifestation of CSCs heterogeneity:

differences in surface marker expression [35].

**3.6. Cancer stem cells (CSCs) dormancy**

CXCR4<sup>−</sup>

mancy, and tumor mass dormancy [19, 40, 41].

expanding beyond a certain number (**Figure 3**).

into the dynamic phase of macrometastatic formation.

[36, 37].

250 Gastric Cancer

arise.

and the CD26<sup>−</sup>

marker expression [39].

**Figure 3.** Metastasis vs. micrometastasis. H&E staining of breast cancer lymph node macrometastases (*A*, ×400) and micrometastases (*B, arrows*, tumor-lymph node interface, ×200). Immunohistochemical analysis of vascularization of human breast cancer lymph node metastases (*C*, ×400) and micrometastases (*D*, ×400). Tumor vascularization was analyzed by staining with polyclonal antibody against factor VIII, an endothelial-speci c marker. In breast cancer metastases (*C*), there was marked neovascularization (brown stain; arrows, representative blood vessels). In contrast, breast cancer micrometastases (*D*) had a marked decrease in tumor microvessel density. *Arrows*, tumor-lymph node interface. Immunohistochemical analyses of proliferation of breast cancer metastases (*E*, ×400) and micrometastases (*F*, ×400). Tumor proliferation was analyzed by staining with antibody against Ki-67. In breast cancer metastases, there was a much higher rate of proliferation (*E*, red/brown stain; arrows, representative proliferating cells) compared with micrometastases (*F, arrows*, tumor-lymph node interface) [71].

characteristics of CSCs so that they can generate or regenerate the tumor bulk and maintain their self-renewing potential. Intracellular and intercellular signals operate within CSC microenvironments and support CSC activities. The internal signals include molecular pathways that regulate stemness, whereas extracellular signals consist of cells designed to anchor CSCs within the microenvironment, and cell receptors and secreted factors that are necessary for maintaining CSCs in their quiescent state [42].

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 [42]. Several intracellular signaling pathways may be altered in the process of malignant transformation of stem cells. For example:


Activated STAT3 has been found in leukemia, squamous cell carcinoma of the head and neck, multiple myeloma, breast cancer, and prostate cancer. Blocking 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 [48].

Cancer-associated cells in the microenvironment may secrete growth factors and cytokines to support CSCs. 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 [49].

During dormancy, micrometastases are somehow able to evolve and acquire a full complement of metastasis-colonization functions that they did not express before. It is difficult to envision how this progression could occur in CSCs (section of micrometastasis) that remain in a state of replicative quiescence. Although CSCs in bone marrow look quiescent, the overall CSCs population is not static. Circulating cancer cells can be detected in blood in the apparent absence of active metastatic disease. If not in the bone marrow, at least in other tissues, micrometastases may be constantly exiting and re-entering a dormant state, and become familiar with the environment, undergoing further selection for colonization traits during the active interludes. Transition between quiescent and proliferative states is a property of adult stem cells that may be hijacked by CSCs [49].
