Recent Advances in Angiogenesis and ... - Bentham Science

113 Recent Advances in Angiogenesis and Antiangiogenesis, 2009 Taraboletti and Bonezzi
effect on angiogenesis is extremely complex and
context-dependent. Like other matricellular proteins, it
is composed of a succession of different functional
domains, each able to specifically interact with cell
receptors, soluble cytokines and growth factors,
extracellular matrix components, and proteases. The
local presence of these receptors/ligands and the
bioavailability of each TSP-1 domain will therefore
dictate the pattern of molecular interactions, hence the
biological effect of TSP-1 in a given biological
setting. Although TSP-1 is commonly considered an
inhibitor of angiogenesis, it can also stimulate it,
through biologically active pro-angiogenic domains,
mostly mapped in the N-terminal domain [15-20].
Thus, TSP-based antiangiogenic therapies could be
designed either to mimic the molecule’s
antiangiogenic properties or to antagonize its proangiogenic
activity.
TSP-1 can affect angiogenesis both directly and
indirectly. As a direct inhibitor, it interacts with
specific receptors on endothelial cells to affect cell
viability and functions related to angiogenesis. As an
indirect inhibitor, it binds to and influences the
activity/bioavailability of various mediators of
angiogenesis, such as angiogenic factors, cytokines
and proteases.
Although this review will mostly focus on the effects
of TSP on endothelial cells, TSP-1 is also active on
other cell types involved in angiogenesis, including
smooth muscle cells, monocytes/macrophages and T
cells [21, 22].
Here we focus on TSP-1 and tumor-associated
angiogenesis. A brief description of membrane
receptors or soluble TSP-1 ligands is provided,
followed by a list of active sequences and the evidence
that these TSP-peptides are bona fide inhibitors of
angiogenesis and tumor growth.
2. “DRUGGABLE” TSP-1 RECEPTORS
AND LIGANDS
TSP-1 interacts with a variety of receptors on the
surface of endothelial cells, eliciting both anti- and
pro-angiogenic responses. Therapeutic exploitation
could therefore by achieved by TSP-1 mimetics in the
first case, and by antagonists in the latter.
Membrane Receptors
CD36, the first TSP-1 membrane receptor identified
[23, 24], is an 88 kDa glycoprotein expressed by
platelets and many cell types, including microvascular
but not large-vessel endothelial cells [25].
The TSP-1/CD36 interaction occurs between the
conserved CLESH-1 domain in CD36 and the type I
repeats in TSP-1 [26]. Homology searches indicate
that the TSP-1 recognition motif of CD36 is found in
several molecules, within and outside the CD36
family, including lysosomal integral membrane
protein II (LIMPII) [27] and HIV envelope gp120 [28].
On the other hand, a large family of proteins - the
TSR-superfamily - contains sequences similar to the
type I repeats [29]. It is therefore plausible that the
CLESH/TSR pattern of recognition extends beyond
the CD36/TSP-1 system and is implicated in different
molecular networks and biological processes.
CD36 mediates the inhibitory effects of TSP-1 on
FGF-2-induced endothelial cell migration,
morphological organization [26, 30] and angiogenesis
in the cornea assay [31]. By preventing free fatty acids
uptake by CD36, TSP-1 causes inhibition of eNOS
and therefore the NO-dependent response [32]. TSP-1
binding to CD36 induces endothelial cell apoptosis
through the sequential activation of p59fyn, caspase-3
and p38 MAPK [31]. Apoptosis ultimately involves
death receptors, the Fas/FasL (CD95/CD95L) or the
TNFa/TNF-R1 systems, with possible differences in
endothelial cell types and stimuli [33, 34].
Interestingly, the TSP-1/CD36 interaction affects the
activity/expression of other receptors involved in
angiogenesis. TSP-1, via CD36, down-modulates
vascular endothelial growth factor (VEGF) receptor 2
(VEGFR-2) [30]. CD36 is also constitutively
associated with ß1 integrin in endothelial cells [30],
with interesting implications for the mechanisms of
the anti-motility activity of TSP-1 and the possibility
of cooperative antiangiogenic activity between integrin
antagonists and CD36-binding sequences of TSP-1.
CD47, also known as IAP (integrin-associated
protein), was initially identified as the cell membrane
receptor for the adhesive sequence 4N1
(KRFYVVMWKK) in the C-terminal G domain of
TSP-1, mediating TSP-1-induced cell spreading [35,
36]. CD47 forms a signaling complex with vß3 and
other integrins (reviewed in [21, 37]). The soluble
4N1 peptide reportedly inhibits tube formation,
though not proliferation, of capillary endothelial cells,
and FGF-2-induced neovascularization in the cornea
assay [38].
The TSP-1/CD47 interaction is important in the
inhibition of nitric oxide (NO) by TSP-1. Although
both CD47 and CD36 contribute to NO regulation by
TSP-1, only CD47 has proved necessary for this effect,
since ligation of CD36 was unable to block NO
signaling in cells lacking CD47 [32]. Regulation of
NO signaling by TSP-1 has important consequences
on the regulation not only of angiogenesis, but also of
tumor blood flow, hence potentially on the activity of
vasoactive and vascular targeting agents [39].
The N-terminal domain of TSP-1, mostly responsible
for the pro-angiogenic activity of the molecule [17-20],
binds heparin and related molecules, such as
heparansulfate proteoglycans and sulfatides. The Nterminal
domain of TSP-1 interacts with syndecan-4
to stimulate endothelial cell tubulogenesis and protect
cells from apoptosis [40]. Syndecan-4 binds two TSP-