**6.1 Niche of LSCs**

*Advances in Hematologic Malignancies*

apoptosis of drug-resistant CD34+

FAS protein [73].

**5.2 NF-κB pathway**

tal effect is remarkable [76, 77].

**5.3 PI3K/Akt pathway**

normal HSC failure.

pro-apoptotic proteins (such as BAX, BAD, and BAK). The ratio between the two types of proteins will directly affect the stability of the mitochondrial membrane and is the most important regulator of the mitochondrial apoptosis pathway. Overexpression of BCL-2, an increase in the ratio of BCL-2 to BAD, leading to defects in mitochondrial apoptosis, is one of the important mechanisms for LSC multidrug resistance and poor prognosis of AML [68]. BCL-2 is the target gene downstream of the Hh pathway, and Hh pathway blockers can induce apoptosis by downregulating BCL-2 [69]. Kobune et al. found that cyclopamine induces

them sensitive to Ara-C [70]. MCL1 has also emerged as a mechanism of resistance to apoptosis and to BCL-2/BCL-XL inhibitors, and therefore, it is considered as a potential therapeutic target [71]. (2) Regulation of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) mediated apoptosis. TRAIL-R3 is a blank recep-

LSCs; the downregulation of TRAIL-R3 increases apoptosis [72]. (3) Regulation of FAS protein express in the death receptor pathway. Studies have found that the Hh pathway inhibitor GDC-0449 promotes tumor stem cell apoptosis by upregulating

NF-κB is a significant transcriptional activator located upstream of the IRF-1 gene. It is aberrantly activated by LSCs. NF-κB not only inhibits apoptosis but also regulating the expression of cytokine genes. Furthermore, apoptosis can be inhibited by inducing and upregulating antiapoptotic genes. Therefore, NF-κB plays an essential role in maintaining LSC growth and survival. Inhibition of this signaling pathway not only promotes LSC apoptosis but also enhances the sensitivity of LSCs to chemotherapeutic drugs [74, 75]. At present, the targeted drugs for NF-κB are mainly proteasome inhibitors MG-132 and Bortezomib (VELCADE, PS-341), which can better target LSCs without any significant effect on normal HSC. It was reported that PTL can specifically induce apoptosis of LSCs by inhibiting NF-κB activity. At present, the PTL analog DMAP has been developed, and its experimen-

The PI3K/Akt pathway is an intracellular pathway that plays a critical role in apoptosis and cancer, whose components are often altered in cancer, leading to dysregulated apoptosis and chemoresistance [78]. Chen et al. demonstrated that the PI3K inhibitor LY294002 can directly target LSCs without adverse reactions to normal HSCs, and they found that PI3K and NF-κB may coexist in the same signaling pathway [79]. Further, it has been reported that the mammalian target of rapamycin (mTOR) is a substrate for PI3K that regulates the survival of LCSs after etoposide treatment. Mise et al. showed that the inhibitory effect of rapamycin on mTOR significantly reduced the survival rate of AML cells, and rapamycin enhanced the effect of etoposide on these cells [80]. It is found that PTEN that negatively regulates the PI3K pathway and is essential for maintaining normal hematopoiesis [81]. However, PTEN deletion has no significant effect on HSC differentiation survival, while PTEN deletion in LSCs can lead to the production and proliferation of LSCs. In addition, rapamycin, an inhibitor of the PI3K pathway downstream regulator of mTOR was found to inhibit LSCs and protect against

tor that lacks a functional death region and is highly expressed in CD34<sup>+</sup>

AML cells by downregulating BCL-2 and makes

CD38<sup>−</sup>

**18**

Niche is involved in stem cell self-renewal, survival, chemotherapy tolerance, and metastasis of leukemia cells [82]. In the mice model, it was found that the homing of HSCs to the bone marrow is regulated by chemokine CXCL12 expressed in mesenchymal stem cells, and its receptor is CXCR4 [83]. Inhibition of CXCL12- CXCR4 interaction helps to reduce chemotaxis, thus affecting the movement, adhesion, and metastasis of LSCs. In vitro studies have shown that the anti-leukemia active peptide CXCR4 inhibitor LY2510924, as a single agent or in combination chemotherapy, can rapidly and permanently destroy the CXCL12-CXCR4 axis, thereby inhibiting the proliferation of AML cells and leading to apoptosis [84]. Fully human IgG4 monoclonal antibody BMS-936564 against CXCR4 showed high safety and antitumor activity in relapsed and refractory patients with AML [85]. However, because of the similar biological properties of LSCs and HSCs, the non-selection of related inhibitors has become another major clinical problem.

In addition to participating in the hematopoietic function, the bone marrow niche is also an important place for the presence of immune cells. There is a group of activated leukemia-specific immune cells in leukemia bone marrow, and regulatory T cell (Treg) is one of the important members [86]. Fujisaki et al. found that hematopoietic stem/progenitor cells and Treg can coexist on the endosteum of murine bone marrow, and HSPC disappears shortly after Treg cell depletion [87]. This experiment successfully demonstrated the involvement of Treg cells in the formation of bone marrow niche. Treg is a dynamic cell population that regulates the immune response. Stem cells evade immune surveillance by recruiting Treg cells and using their regulatory functions [88]. Therefore, it is speculated that these cells will likely become new targets for eliminating LSCs (**Figure 2**) [89].
