**4. The role of LSCs in tumor metastasis and invasion**

CSCs are thought to be the seed of tumor metastasis. CSCs that particularly express C-X-C chemokine receptor type 4 (CXCR4) preferentially disseminate [46]. The specific ligand for the CXCR4 chemokine receptor is termed matrix-derived factor-1 (SDF-1, also known as CXCL12). Both CXCR4 and SDF-1 are expressed in various tissues and cell types and regulate cell migration [47]. The SDF-1/CXCR4 axis is also involved in the migration of CSCs [48]. SDF-1 is a homeostatic chemokine secreted by stromal cells and is released into the interstitial space [49]. On the one hand, SDF-1 exerts effects through its unique physiologic cognate receptor CXCR4, which is known to mediate chemotaxis, hematopoiesis, angiogenesis, and tumor spread and metastasis. On the other hand, it also acts in a paracrine fashion on cells in the local microenvironment to stimulate directional migration of hematopoietic and nonhematopoietic normal and malignant cells [50–52]. Li et al. found that HERG K+ channels were widely expressed in primary leukemic cells but not in normal lymphocytes [53, 54]. Blocking HERG K+ channels by applying its specific inhibitor in hematopoietic cell lines and primary leukemic cells significantly reduced the migration of leukemic cells induced by SDF-1; this indicated a role for HERG K+ channels in the progression of leukemia.

**17**

*Advances in Acute Myeloid Leukemia Stem Cells DOI: http://dx.doi.org/10.5772/intechopen.84263*

and migration, was significantly higher in MOLT4<sup>+</sup>

resistant MOLT4+

known to aberrant activate LSCs.

cell proliferation and survival [64–66].

**5.1 Hedgehog pathway**

Currently, there is a lack of direct evidence linking LSCs to metastasis. There are some sporadic reports that LSCs may play a role in metastasis. In patients with AML, low levels of CXCR4 expression have been shown to be associated with better prognosis, longer recurrence-free period, and overall survival. It has also been suggested that CXCR4 is an independent prognostic predictor of disease recurrence and survival [55]. Another study has shown that overexpression of C-myc, Bmi-1, Oct4, and Nanog in precancerous and cancerous cells may initiate oncogenic epithelial-mesenchymal transition and tumorigenesis, which plays important roles in the genesis of CSCs, malignant tumor initiation and progression, cancer metastasis, and drug resistance [56]. Compared with the parental cells, chemotherapy-

CXCR4. It was found that the expression of CXCR4, related to tumor cell homing

addition, hMDSCs-MOLT4 cells seem to have a strong invasive potential in vivo, as demonstrated by strong interstitial and vascular tissues in tumor tissue sections. It was confirmed that the niche was involved in metastasis. With respect to HSCs, two distinct niches have been defined: the osteoblastic niche and the vascular niche [57–59]. Tabe et al. hypothesized the presence of a "metastatic niche" that facilitates the survival, proliferation, and metastasis of LSCs [60]. Yang et al. demonstrated that vascular endothelial growth factor receptor 1 (VEGFR1) was involved in the initiation of a premetastatic niche and that cells expressing VEGFR1 home to tumor-specific premetastatic sites and form cellular clusters before the arrival of tumor cell clusters [61]. They can alter the local microenvironment and lead to the activation of integrins and chemokines. After treatment with anti-VEGFR1 antibodies, the supportive premetastatic cell clusters were abolished and metastasis was prevented, which indicated the importance of a metastatic niche.

**5. The role of LSCs in tumor proliferation and anti-apoptosis**

Various signaling pathways that stimulate proliferation or inhibit apoptosis are

The Hh pathway is a highly conserved pathway that regulates the proliferation, migration, and differentiation of cells during development [62, 63]. Three distinct ligands, namely Sonic (Shh), Indian (Ihh), and Desert (Dhh) Hedgehog, exist in humans. Upon ligand binding to the receptor pat (Ptch), inhibition of smoothened (Smo) receptor is relieved. Smo then activates members of the Gli family of zinc-finger transcription factors, translocating them to the nucleus to regulate the transcription of Hh target genes including Gli1, Gli2, and Ptch, and regulators of

The Hh pathway promotes cell proliferation mainly by regulating cell cycle. Its regulation mechanism is as follows [67]: (1) Cyclin D1 and cyclin D2 act as downstream target genes for the transcription factor GLI1 and are involved in cell cycle G1 to S phase transformation; (2) PTCH regulates the activity of cyclin B, which is part of the mitosis promoting factor (MPF) compound. MPF is required for cell entry from the G2 phase to the M phase; and (3) SMO proteins block cellular

The Hh signaling pathway regulates apoptosis mainly through the following mechanisms: (1) Regulate the activity of the BCL-2 family. The BCL-2 family is divided into anti-apoptotic proteins (such as BCL-2, BCL-XL, and MCL-1) and

dormancy by modulating P21, a cyclin-dependent inhibitory protein.

cells expressed much higher levels of the stem cell surface marker

cells than in MOLT4<sup>−</sup> cells. In

*Advances in Acute Myeloid Leukemia Stem Cells DOI: http://dx.doi.org/10.5772/intechopen.84263*

*Advances in Hematologic Malignancies*

for LSC survival maintenance [41, 42].

**3.4 The special microenvironment (niche)**

**3.5 Multiple signaling pathway abnormalities**

miR-126 levels [38]. Moreover, leukemic cells with high miR-126 expression were selected in refractory patients after induction chemotherapy, thus correlating high miR-126 levels to LSCs and therapy resistance. miR-126 knockdown leads to the expansion of HSCs but impaired maintenance of LSCs, and its overexpression promotes LSC self-renewal, which is inhibited in HSCs [39, 40]. In addition, all genes and signaling pathways that contribute to HSC self-renewal may be involved in LSCs, such as Wnt, Notch, HOX, and Shh. Recent studies have revealed that the activation of the Shh signaling pathway in LSCs by upregulation of SMO is essential

The receptors CXCR4 on the LSC membrane and CXCL12 in the bone marrow microenvironment are required for LSC to maintain dormancy, self-renewal, differentiation, growth, and homing. However, targeted therapy for the niche will enhance the expression of the drug pump MDR1, which induced LSC insensitive to

Recent studies have demonstrated that abnormal activation of multiple signaling pathways is one of the key mechanisms of LSC multidrug resistance, such as Sonic Hedgehog, Bmi-1, Nocth, and WNT. Among these pathways, the abnormality of Hedgehog (Hh) pathway is closely related to CSC resistance, such as increased endogenous synthesis of ligand protein Hh, loss of PCTH activity, inhibition of smoothened (SMO) signaling protein, mutation of SUFU, and overexpression of the transcription factor GLI1, thus regulating the downstream target gene and participating in the maintenance of stem cell proliferation, which are related to multiple hallmarks of tumor cell resistance [44, 45]. Studies have revealed that Hh signaling is abnormally activated in LSCs, GLI1 can induce endogenous BCL-2 expression, and the Hh pathway also up-regulates BCL-2 by activating PI3K/AKT,

CSCs are thought to be the seed of tumor metastasis. CSCs that particularly express C-X-C chemokine receptor type 4 (CXCR4) preferentially disseminate [46]. The specific ligand for the CXCR4 chemokine receptor is termed matrix-derived factor-1 (SDF-1, also known as CXCL12). Both CXCR4 and SDF-1 are expressed in various tissues and cell types and regulate cell migration [47]. The SDF-1/CXCR4 axis is also involved in the migration of CSCs [48]. SDF-1 is a homeostatic chemokine secreted by stromal cells and is released into the interstitial space [49]. On the one hand, SDF-1 exerts effects through its unique physiologic cognate receptor CXCR4, which is known to mediate chemotaxis, hematopoiesis, angiogenesis, and tumor spread and metastasis. On the other hand, it also acts in a paracrine fashion on cells in the local microenvironment to stimulate directional migration of hematopoietic and nonhematopoietic normal and malignant cells [50–52]. Li et al. found

inhibitor in hematopoietic cell lines and primary leukemic cells significantly reduced the migration of leukemic cells induced by SDF-1; this indicated a role for

channels were widely expressed in primary leukemic cells but not in

channels by applying its specific

therapy and failed to achieve the goal of reversing its resistance [43].

thus leading to apoptotic disorder and drug resistance of LSCs.

**4. The role of LSCs in tumor metastasis and invasion**

normal lymphocytes [53, 54]. Blocking HERG K+

channels in the progression of leukemia.

**16**

that HERG K+

HERG K+

Currently, there is a lack of direct evidence linking LSCs to metastasis. There are some sporadic reports that LSCs may play a role in metastasis. In patients with AML, low levels of CXCR4 expression have been shown to be associated with better prognosis, longer recurrence-free period, and overall survival. It has also been suggested that CXCR4 is an independent prognostic predictor of disease recurrence and survival [55]. Another study has shown that overexpression of C-myc, Bmi-1, Oct4, and Nanog in precancerous and cancerous cells may initiate oncogenic epithelial-mesenchymal transition and tumorigenesis, which plays important roles in the genesis of CSCs, malignant tumor initiation and progression, cancer metastasis, and drug resistance [56]. Compared with the parental cells, chemotherapyresistant MOLT4+ cells expressed much higher levels of the stem cell surface marker CXCR4. It was found that the expression of CXCR4, related to tumor cell homing and migration, was significantly higher in MOLT4<sup>+</sup> cells than in MOLT4<sup>−</sup> cells. In addition, hMDSCs-MOLT4 cells seem to have a strong invasive potential in vivo, as demonstrated by strong interstitial and vascular tissues in tumor tissue sections.

It was confirmed that the niche was involved in metastasis. With respect to HSCs, two distinct niches have been defined: the osteoblastic niche and the vascular niche [57–59]. Tabe et al. hypothesized the presence of a "metastatic niche" that facilitates the survival, proliferation, and metastasis of LSCs [60]. Yang et al. demonstrated that vascular endothelial growth factor receptor 1 (VEGFR1) was involved in the initiation of a premetastatic niche and that cells expressing VEGFR1 home to tumor-specific premetastatic sites and form cellular clusters before the arrival of tumor cell clusters [61]. They can alter the local microenvironment and lead to the activation of integrins and chemokines. After treatment with anti-VEGFR1 antibodies, the supportive premetastatic cell clusters were abolished and metastasis was prevented, which indicated the importance of a metastatic niche.
