Recent Advances in Angiogenesis and ... - Bentham Science

12 Recent Advances in Angiogenesis and Antiangiogenesis, 2009 Leali and Naldini
OPN acts also as a substrate for matrix
metalloproteases MMP-3 and MMP-7, and the cleaved
fragments enhanced adhesion and migration in vitro
through ligation of receptors including 1integrin [54].
Indeed, there are two different binding sites for 41
integrin present in a 38-amino acid domain within the
N-terminal thrombin fragment, corresponding to both
162 SVVYGLR 168 and 131 ELVTDFPTDLPAT 143 motifs
[55]. Furthermore, the sequence 43 WLNPDP 48 has
been recently described as a novel functional motif of
OPN, involved in migration and survival of human
lymphocyte, although the corresponding interacting
receptor has not been identified [56].
A
17
HN 2
N-terminus
131 ELVTDFPTDLPAT 143
a1b1 158 168
GRGDSVVYGLR
a1b1,
a1b1 a1
b1,
a1
b1
a b a b
1 1, 1 1
Thrombin
168 169
R
a1b1
a1b1
a b
1 1
C-terminus
CD44v6, CD44v6-v7,
CD44v3, (heparin bridge)
MMP-3,7
1 MRIAVICFCLLGITCAIPVKQADSGSSEEKQLYNKYPDAV
41 ATWLNPDPSQKQNLLAPQNAVSSEETNDFKQETLPSKSNE
Fig. (1). OPN protein structure. (A) Schematic
representation of the human OPN protein. OPN contains
a thrombin cleavage site that separates the Nterminus/integrin
binding and the C-terminus/CD44v
binding domains (amino acid residues 17-168 and 169-
314, respectively). Conserved regions involved in
receptor interactions are indicated. (B) Amino acid
sequence (single letter code) of the human OPN protein
(GenBank accession number: J04765). The N-terminus
and C-terminus domains are underlined and shown as
dotted line, respectively. The signal sequence (amino acid
residues 1-16) is in Italics. Arrow indicates the thrombin
cleavage site.
In keeping with its ability to associate with different
ECM proteins, such as collagen and fibronectin
[57,58], a novel functional domain, spanning both the
SK
314
COOH
81 SHDHMDDMDDEDDDDHVDSQDSIDSNDSDDVDDTDDSHQS
121 DESHHSDESDELVTDFPTDLPATEVFTPVVPTVDTYDGRG
161 DSVVYGLRSKSKKFRRPDIQYPDATDEDITSHMESEELNG
201 AYKAIPVAQDLNAPSDWDSRGKDSYETSQLDDQSAETHSH
241 KQSRLYKRKANDESNEHSDVIDSQELSKVSREFHSHEFHS
281 HEDMLVVDPKSKEEDKHLKFRISHELDSASSEVN
B
NH2-terminal and the C-terminal regions of the
protein, has been recently identified as “collagen
binding motif”, corresponding to the
166 GLRSKSKKFRRPDIQYPDATDEDITSHM 193
sequence in human OPN [59].
The C-terminal fragment of OPN contains a conserved
calcium binding site and interacts directly with
CD44v6- and v7-containing isoforms [44,60],
although CD44v3 might indirectly bind OPN through
a heparin bridge [52]. This RGD-independent
interaction appears to require the presence of 1
integrins [61]. It has been recently proposed a model
for human OPN structure in which a sequence in the
C-terminal region forms a -sheet structure with the
RGDSVVYGLR domain in the N-terminus, thus
interfering with RGD-integrin interaction [46]. This
model might explain the ability of monoclonal
antibody against the C-terminus domain to inhibit cell
adhesion to OPN, thus suggesting the possibility that
CD44/OPN interaction modulates the cells’ capacity to
recognize the RGD sequence [62].
Although expressed as a ~ 33 kDa nascent protein,
extensive posttranslational modifications (PTMs) such
as Ser/Thr phosphorylation, O-linked/N-linked
glycosylation, sialylation, Tyr sulfation and enzymatic
cleavage, increase its apparent molecular weight, thus
determining a protein migration in SDS-PAGE gels in
the range of 44-80 kDa depending on conditions
[24,63]. Polymeric form of OPN has been recently
reported which produces a band around 200 kDa that
is mediated by transglutaminase [64,65]. Many sites of
PTMs are conserved across species; however, the
degree of modification of the protein varies depending
on the source tissue and cell type or differentiation
stage [66-68] and influence OPN function [46].
Phosphorylation of OPN appears necessary for various
physiological functions, including migration of cancer
cells [69], adhesion and bone resorption by osteoclasts
[70], inhibition of smooth muscle cell calcification
[71] and regulation of mineralization [72]. The
phosphorylation of OPN is usually heterogeneous, and
it is not known whether certain specific sites are
critical for a given function; furthermore, it is possible
that differences in OPN post-translational modification
status, together with variations in the target cell
receptor repertoire, modulate OPN’s functions or the
cellular response to OPN [46].
4. OPN SIGNALING
The role of both secreted and intracellular OPN in cell
signaling has been recently reviewed in the context of
cancer progression and immune response [40,73].
Many of the signaling pathways mediated by secreted
OPN are activated by ligation of the integrin and
CD44 families of receptors. OPN signaling through