H e m a t o lo g y E d u c a t io n - European Hematology Association

potential to prevent LSCs from accessing such sanctuary
sites, or to mobilize them from these sites, and has
received attention as a novel therapeutic strategy to resensitize
LSCs to TKIs.
CXCR4
The most comprehensively investigated target to date
is the CX chemokine CXCL12 signaling pathway.
CXCL12 is expressed and secreted by stromal cells
within the bone marrow, signals solely through the
membrane receptor CXCR4, and has been reported to
control HSC chemotaxis and quiescence. 45,46 CML
CD34+ cells from chronic phase and blast crisis patients
express significantly lower levels of CXCR4 than their
normal counterparts, correlating with impaired migratory
capacity towards CXCL12. 47 Jin et al. confirmed
CXCR4 expression to be negatively regulated by BCR-
ABL1, and restored to normal levels on imatinib treatment
of blast crisis CML patients in vivo, leading to the
hypothesis that TKI treatment may increase the ability
of LSCs to access the stem cell niche. In support of this,
imatinib treatment increased migration of CML cell
lines towards CXCL 12/SDF-1 in vitro and elevated
CXCR4 expression on CD34+ cells within the bone
marrow of CML patients. 47 Preliminary work within our
laboratory suggests that the CXCR4 antagonist plerixafor
(AMD3100) sensitizes CML stem cells to TKI treatment
in vitro, 48 and in vivo studies are ongoing. 49
JAK
In addition to chemokines, cytokines are also implicated
in BCR-ABL1-independent maintenance of CML
stem cells. Hiwase et al. recently reported that a cocktail
of Flt3-ligand, stem cell factor, interleukin (IL)-3. IL-6,
and granulocyte colony-stimulating factor promotes
survival of CML LSCs in the presence of TKI treatment.
50 This group, therefore, investigated a small molecule
Janus tyrosine kinase (JAK), a cytokine signaling
intermediary inhibitor, and demonstrated that it re-sensitized
CML CD34+ cells to dasatinib in the presence of
growth factors, 50 suggesting this class of agent to be
worthy of further study.
Autophagy
Autophagy has also been shown to play a role in the
survival of CML stem cells in response to TKIs.
Autophagy is a cell survival process that mediates the
breakdown of intracellular material to protect the cells
from cell death during times of stress, such as starvation,
which leads to loss of growth factor signaling. 51 Bellodi
and colleagues noted that CML cell lines that survive
TKI have an increased number of cytoplasmic vacuoles,
an indicator of autophagy. Subsequent investigation
confirmed the induction of autophagy in response to
imatinib in CML cell lines and primary CML cells, an
effect that appeared to be mediated by imatinibinduced
ER stress. While autophagy can result in Type
II cell death, in CML cells, it clearly induced a survival
response as inhibition of autophagy by chemical or
genetic means potentiated imatinib-induce cell death.
Importantly, treatment of primary CML cells with TKIs
in combination with the autophagy inhibitor chloro-
quine resulted in near ablation of the CML stem cells. 28
This suggested that inhibiting autophagy in combination
with TKI treatment could be a means of eradicating
CML 52 and has consequently led to the CHOICES Phase
II clinical trial combining hydroxychloroquine with
imatinib (ClinicalTrials.gov NCT01227135).
LSC versus HSC
London, United Kingdom, June 9-12, 2011
Exploiting normal stem cell signaling pathways as
therapeutic targets will inevitably impact on the normal
HSC compartment. As within chronic phase CML
patients, normal HSCs are reported to greatly outnumber
LSCs. 53 A therapeutic window may exist for these
novel therapeutic approaches to translate successfully to
the clinic, so a degree of selectivity for LSCs is desirable.
As BCR-ABL1 activity per se does not seem to be so
important in the LSC compartment, targeting other proteins/pathways,
which are deregulated in LSCs compared
with normal HSCs, may be a means of eradicating
this persistent population. A number of proteins
have already been identified which could be targets for
therapy.
Alox5
Alox5, the gene that transcribes for Arachidonate 5lipooxygenase
(5-LO), was shown to be upregulated in
BCR-ABL1+ve LSCs but was not modulated by imatinib,
suggesting its expression to be independent of
BCR-ABL1 kinase activity. 54 This enzyme, which converts
arachadonic acid into leukotrienes 55 and its products,
has been shown to be upregulated in a number of
cancers and is thought to be involved in proliferation
and survival of tumor cells. 56 Inhibition of 5-LO in blast
crisis CML cells in vitro resulted in an inhibition of proliferation
and induction of apoptosis, 57 while knockout
of 5-LO or inhibition with zileuton in a mouse model of
CML affected LSC survival and inhibited CML development.
54 These results suggested that zileuton, which is
already in clinical trials for the treatment of asthma,
could be a novel therapy that would specifically target
LSCs but not normal HSCs. 54 Indeed, there is an ongoing
Phase I clinical trial looking at the safety of zileuton in
combination with imatinib in CML patients.
PP2A
Another protein that is deregulated in CML is PP2A
(protein phosphatase 2A). PP2A is a serine threonine
phosphatise, which has been shown to have a role in
cell survival, proliferation, and differentiation, and acts
as a tumor suppressor protein. 58) In CML chronic and
blast crisis CD34+ cells, PP2A’s activity was inhibited
through upregulation of the phosphoprotein SET by
BCR-ABL1. This inhibition was associated with disease
progression and appeared to require increased phosphorylation
of PP2A at tyrosine 307. Interestingly BCR-
ABL1 activity itself was regulated by PP2A, and re-activation
of PP2A interfered with the leukemogenic activity
of BCR-ABL1. 59 Indeed an activator of PP2A, FTY720,
which is already in Phase III clinical trials for multiple
sclerosis, was shown to have efficacy in the treatment
of blast crisis CML and BCR-ABL1 positive ALL in vitro
and in vivo. 60 In addition, recent evidence suggests the
Hematology Education: the education programme for the annual congress of the European Hematology Association | 2011; 5(1) | 115 |