Endothelial Progenitor Cells at Work
Not Mature Yet, but Already Stress-Resistant
Lothar Rössig, Carmen Urbich, Stefanie Dimmeler
T
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issue replacement in the adult organism by cell-specific
differentiation of autologous stem/progenitor cells has
evolved as a fascinating concept in stem cell biology. After
organ damage, bone marrow– derived circulating or tissueresident progenitor cells are thought to differentiate toward
the type of cell needed for repair. According to this concept,
maturation of these cells would be expected to result at best
in a perfect morphological and functional replacement of the
injured tissue. However, in this issue of Arteriosclerosis,
Thrombosis, and Vascular Biology, He et al show that
endothelial progenitor cells (EPCs) are more than just as
capable of in vitro angiogenic tube formation as mature
endothelial cells (ECs), but are truly advantageous when it
comes to stress tolerance.1
ischemia, the tissue pH decreases, and proteolytic enzymes
are released from damaged lysosomes of necrotic cell remnants. At infarct border zones, reperfusion results in the
accumulation of reactive oxygen species (ROS) and, subsequently, of oxidized metabolites. Eventually, invading neutrophils and macrophages contribute additional oxygen radicals (Figure). Thus, for the purpose of recreating a perfusion
network in a postischemic environment, angiogenic precursor
cells first of all have to withstand oxidative stress. Such
reflections may have been made by He et al to ask the
question whether the superiority of EPCs to perform regenerative neovascularization in a temporarily nonperfused tissue could be caused by an enhanced cytotoxic resistance. To
test this hypothesis, they compare viability and function of
umbilical vein and mature coronary ECs versus EPCs after
exposure toward the naturally occurring ROS-generating
cytokine tumor necrosis factor (TNF) ␣ and a synthetic
superoxide generator. The results are astonishing: EPCs
display a unique robustness against ROS-driven cytotoxicity.
Not only is cell viability preserved, but also the angiogenic
capacities of EPCs are fully protected against the ROS
assault. When tracking down the pathways of antioxidant
resistance signaling, He et al come across the enzyme family
of superoxide dismutases (SOD). Among those, they identify
manganese SOD (MnSOD) as the mediator of survival, which
is expressed at highest levels in EPCs compared with mature
ECs. The protective relevance of this enzyme is demonstrated
by the capacity of adenoviral overexpression of MnSOD to
largely enhance the resistance of mature ECs toward superoxide generation close to the levels of EPCs.
These data give new insight into the functional capacities
of EPCs to protect themselves against the rough conditions
under which they have to work to recreate a capillary network
(Figure). Indeed, expression of MnSOD at high levels may
maintain EPC survival and thus explain their extraordinary
capacities for postischemic neovascularization. However, 2
recent studies additionally suggested that an increased resistance against stress could exemplify a rather general stem cell
feature beyond the protection against apoptosis. Transcriptional profiling of mouse embryonic as well as adult neural
and hematopoietic stem cells revealed a characteristic pattern
of so-called “stemness genes,” the expression of which is
typical for these cells and distinguishes them from mature
cells.9,10 Among other gene families, a set of stress response
genes involved in the redox balance were identified to be a
characteristic feature of stem cells, proposing that one essential attribute of “stemness” includes the high resistance to
stress.9,10 These descriptive studies, however, did not address
what is functionally controlled by redox regulatory enzymes
in stem cells. Observations in neural progenitor cells have
See page 2021
EPCs were originally characterized as cells that are mobilized
from the bone marrow and circulate in the peripheral blood
and express certain surface membrane markers including the
vascular endothelial growth factor receptor (VEGF-R2) KDR
and the hematopoietic progenitor cell markers CD34 and
CD133.2– 4 During ex vivo expansion, these cells develop
morphological and functional characteristics typical for ECs,
including formation of vascular-like structures in matrigel
and other in vitro angiogenesis assays. Most importantly,
however, transplanted EPCs exhibit an extraordinary potent
in vivo capacity to improve the neovascularization of ischemic tissue in the adult organism.5 In this regard, EPCs were
shown to be more effective than mature ECs in animal
models of hind limb ischemia,6 – 8 although mature ECs are
well established to exert a potent in vitro angiogenic activity.
Thus, the angiogenic capacity of various cell types in ischemic tissue differs from their vascular-like structure forming
activity in in vitro angiogenesis assays.
But what enables EPCs to promote postischemic vessel
growth that much? To understand the advantageous profile of
EPCs for fulfilling the task of neovascularization after ischemia, one has to consider the changes in the tissue environment after ischemia. After anaerobic metabolism during
From Molecular Cardiology, Department of Internal Medicine IV,
University of Frankfurt, Germany.
Correspondence to Stefanie Dimmeler, PhD, Molecular Cardiology,
Dept. of Internal Medicine IV, University of Frankfurt, Theodor SternKai 7, 60590 Frankfurt, Germany. E-mail [email protected]
(Arterioscler Thromb Vase Biol. 2004;24:1977-1979.)
© 2004 American Heart Association, Inc.
Arterioscler Thromb Vasc Biol. is available at http://www.atvbaha.org
DOI: 10.1161/01.ATV.0000146815.54702.75
1977
1978
Arterioscler Thromb Vasc Biol.
November 2004
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Endothelial progenitor cells (EPCs) contribute to postischemic
neovascularization in the adult organism. Ischemic tissue and
ischemia/reperfusion–related injury provide cytotoxic stresses
(left), which may compromise EPC function and survival. To preserve the neovascularizing capacities of EPCs in this environment, EPCs are equipped with a protective set of genes including MnSOD (right).
shown that factors promoting self-renewal cause these cells to
enter a more reduced redox state, whereas exposure to
signaling molecules that promote differentiation leads to an
excess of oxidation in these progenitors.11 Thus, ROS may
regulate the balance between self-renewal and differentiation.
At the same time, maturation processes coincide with changes
in the redox balance,11 suggesting that antioxidant enzymes
could play a major role for the preservation of “stemness.”
According to this hypothesis, one may speculate that the
increased expression of MnSOD in EPCs compared with
mature ECs reported by He et al actively maintains EPCs in
an undifferentiated, self-renewing state to allow for progenitor cell expansion at sites of repair. Additionally, other
functional capacities of EPCs such as migration, which
determines the functional improvement of patients after cell
transplantation,12 might be critically influenced by the redox
equilibrium.13
Although He et al elegantly demonstrate that MnSOD
expression is sufficient for EC protection from cell death, it
remains to be shown whether EPC survival actually depends
on the presence of MnSOD, or whether there are redundant
tools against ROS available. Interestingly, while the homozygous MnSOD-deficient mice die from dilated cardiomyopathy at postnatal day 10,14 even the heterozygous MnSODdeficient mice experience increased oxidative stress in
various tissues.15 In line with the importance of MnSOD in
the in vivo gene deficiency model, Dernbach et al demonstrated that survival as well as the migratory capacity of
EPCs, an important functional property of EPCs required for
capillary sprouting, is reduced after genetic knock-down by
transient transfection of RNA interference oligonucleotides
against MnSOD.13 However, the results of Dernbach et al
also show that only the combined inhibition of catalase,
MnSOD, and glutathione peroxidase significantly decreases
EPC survival and migration in the presence of hydrogen
peroxide.13 From these data, it appears likely that redundant
antioxidant defense strategies warrant maintenance of the
equilibrium and EPC viability.
Thus, the expression of MnSOD is essential for EPC
function. But are all EPCs equally equipped with enough
MnSOD to protect themselves against untimely differentiation and cell death? He et al made use of EPC preparations
from healthy human donors. However, it is exactly the
functional defectiveness of EPCs that could underlie insufficient postischemic regeneration of ischemic tissue in patients
with coronary artery disease.16 –19 Therefore, based on the
data of He et al, it will be mandatory to compare the
expression of MnSOD between EPCs derived from healthy
donors versus patients with increased cardiovascular risk,
manifest coronary artery disease, and/or ischemic cardiomyopathy. Preliminary data indeed indicate that the expression
of MnSOD is reduced in EPCs of patients with coronary
artery disease (C.U., unpublished data, 2004). If the regenerative potential of EPCs in these patients essentially depends
on the expression and activity of MnSOD, one could envision
to overexpress MnSOD therapeutically in these cells to
improve survival and enhance their regenerative potential for
autologous transplantation.
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Endothelial Progenitor Cells at Work: Not Mature Yet, but Already Stress-Resistant
Lothar Rössig, Carmen Urbich and Stefanie Dimmeler
Arterioscler Thromb Vasc Biol. 2004;24:1977-1979
doi: 10.1161/01.ATV.0000146815.54702.75
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