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variant isoforms (CD44v), is a key regulator of LSC homing to BM niches and
maintenance of their primitive state (Jin et al. 2006 ). CD44 modulates interactions
between LSCs and extracellular matrix components and growth factor ligands to
promote CD44/ligand/receptor tyrosine kinase (RTK) complex formation and sig-
nal transduction (Nervi et al. 2009 ). CD44–hyaluronan interactions contribute to
self-renewal, proliferation, differentiation, homing to the BM, and preservation of
the integrity of the stem cell genome by decreasing DNA damage and enhancing
DNA repair (Williams et al. 2013 ). CD44/ligand/RTK signaling may participate in
reprogramming of leukemia cells to exhibit a more stem cell-like LSC phenotype by
modulating microRNA expression to regulate promoter methylation status and gene
expression (Williams et al. 2013 ). Although activities of CD44s or CD44v have
been reported to be similar in hyaluronan-mediated regulation of HSC differentia-
tion and MSC homing to BM (Avigdor et al. 2004 ), CD44v expression preferen-
tially enforces maturation of self-renewing LSCs (Herrlich et al. 2000 ). Cells in the
BM niche express integrins as cell adhesion receptors that link extracellular adhe-
sion molecules with the intracellular actin cytoskeleton (Redondo-Muñoz et al.
2008 ) and are required for lodging of LSCs in the BM niche (Redondo-Muñoz et al.
2008 ). Integrin heterodimers, composed of an 18α subunits and an 8β subunit, regu-
late cell–cell adhesion, growth factor receptor signaling, cell lineage specification,
differentiation, survival, proliferation, and migration (Prowse et al. 2011 ). Many of
these functions parallel CD44 expression, suggesting integrin–CD44 interactions
(Williams et al. 2013 ). Homing of HSCs to the BM requires a coordinated sequence
of four steps, including E-selectin receptor/ligand interaction and engagement of
CXCL12–CXCR4 signaling, resulting in activation of very late antigen–4 (VLA-4;
integrin α 4 β1); VLA-4 adherence to vascular cell adhesion molecule 1 (VCAM-1);
and transmigration on endothelium (Sackstein 2011 ). VLA-4 binds to CD44v to
form a docking complex for pro-matrix metalloproteinase-9, which is associated
with transendothelial migration and invasion of chronic lymphocytic leukemia
(CLL) B-cells (Redondo-Muñoz et al. 2008 ). Conditional deletion of α4 sensitized
BCR-ABL(+) leukemia to nilotinib and pharmacological VLA4 blockade with the
antibody natalizumab prolong survival of NOD/SCID recipients of primary ALL
when combined with chemotherapy, indicating a role for this integrin in chemore-
sistance of lymphoid malignancies (Hsieh et al. 2013 ). Mechanistic in vitro studies
revealed that the interaction between vascular cell adhesion molecule 1 (VCAM-1)
and VLA-4 play an integral role in the activation of NF-κB in both, the stromal and
the tumor cell compartments (Jacamo et al. 2014 ). Nuclear factor-kappa beta activa-
tion via the VCAM-1/VLA-4 interaction causes increased numbers of dysplastic
hematopoietic cells and progression into secondary AML (Rupec et al. 2005 ).
Abnormal ANG1/TIE signaling has been detected both in endothelial cells and leu-
kemia cells (Watarai et al. 2002 ; Wakabayashi et al. 2004 ). In leukemia cells, auto-
crine ANG1/TIE2 signaling activates STAT1/3/5/6 and extracellular regulated
kinase pathways that increase proliferation, and TIE2/IP-3 kinase signaling sup-
ports AML cell survival (Watarai et al. 2002 ; Wakabayashi et al. 2004 ). Secretion of
the proinflammatory cytokines TNF-α and IL-1β by LSCs upregulates endothelial
adhesion receptors such as selectins, VCAM-1, and intercellular adhesion
Y. Tabe and M. Konopleva