Recent Advances in Angiogenesis and ... - Bentham Science

Contribution of Circulating Endothelial Cells Recent Advances in Angiogenesis and Antiangiogenesis, 2009 61
Fig. (2). Overview of cellular events with an EC phenotype enumerated by flow cytometry (from Bertolini et al, 2009,
modified). The majority of CD45-,CD31+CD146+ events are apoptotic or necrotic cellular fragments (1-10/L in the blood).
The use of Syto16 allows the enumeration of DNA-containing CD45-CD31+CD146+ CECs (0.01-1/L). Along with CECs,
TEM showed the presence of some small, viable and lymphoid-like cells that are compatible with a progenitor cell
morphology, lack CD45 expression and may express CD133 and/or CD34 (Mancuso et al, in press).
growth factors such as VEGF, bFGF, HGF and many
others by cancer cells and/or various host cells [8].
The recent and unexpected finding of an autocrine
loop in ECs [35] is of particular interest, because it
might be that the increase of viable CECs in the blood
of cancer patients mirrors an aberrant vascular
turnover/remodeling associated with high local levels
of VEGF produced by cancer cells.
Following the preclinical evidence that CEC count can
be used as a surrogate biomarker for angiogenesis and
anti-angiogenic drug activity by means of determining
the optimal biological dosage of anti-angiogenic drugs
[24-26], their numbers and viability have also been
measured in different clinical trials involving cancer
patients treated with various anti-angiogenic therapies
[10-21]. An increase in the number of apoptotic CECs
after 60 days of therapy was associated with prolonged
progression-free survival and overall survival in
metastatic breast cancer patients treated with a doublet
low-dose metronomic (antiangiogenic) chemotherapy
regimen [16]. When the humanized anti-VEGF
antibody, bevacizumab, was added to the metronomic
chemotherapy for the treatment of metastatic breast
cancer patients who showed a clinical response in a
phase II clinical trial (as well as a larger population of
patients who had a clinical benefit from the treatment)
had significantly greater baseline levels of viable
CECs than did patients who failed to respond;
furthermore, the number of apoptotic CECs before
therapy initiated was associated with prolonged
progression-free survival [36]. In patients with renal
cancer treated with the small molecule anti-angiogenic
agent, sunitinib, changes in CECs differed between the
patients with clinical benefit and those with
progressive disease [18]. Taken together, these data
suggest that the investigation of CEC number and
viability by FC has potential for the stratification of
cancer patients who are more likely to benefit from
anti-angiogenic treatments [8, 25, 37]. This possibility
awaits confirmation in prospective randomized clinical
trials.
4. THE CATALYTIC CEP ROLE IN
DEVELOPMENT OF MACROMETAS-
TASES AND RESISTANCE TO CHE-
MOTHERAPY, VDAS AND ANTI-
ANGIOGENIC DRUGS
Asahara et al first proposed the concept of CEPdependent
new blood vessel formation in adult life in
1997 [27], a few months before a similar proposal
from Shi et al in 1998 [28]. These two seminal studies
identified endothelial progenitors as cells that are
capable of generating mature ECs in vitro and in vivo.
A study from Lyden et al [38] was the first to identify
a role for CEPs in cancer growth using Id (Id1 +/- Id3 -/- )
deficient mice. These mice have a severe CEP defect
leading to impaired angiogenesis and tumor growth.
More recently it was found that CEP generation
depends on the ability of Id to restrain the expression
of its target gene p21 [39]. Genetic ablation of p21
rescued CEP generation in Id1 deficient mice, reestablishing
normal tumor growth [39]. Understanding
whether all types of tumor rely, at least in part, on