ECM Remodeling in Angiogenesis - Mechanical and Chemical Signaling in Angiogenesis - page 193

TIMP-3, as briefly mentioned earlier, can inhibit both MMPs and the ADAM
family of proteinases [ 40 ]. TIMP-3 is also known to block VEGF to VEGFR-2
binding, which further contributes to the anti-angiogenic capabilities of TIMP-3
[ 99 ]. A third unique property of TIMP-3 is its restricted diffusion caused by its
tight binding to heparin sulfate proteoglycans in the surrounding ECM. Because it
does not readily diffuse, it is thought that TIMP-3 instead functions to regulate
angiogenesis after the angiogenic switch has been flipped into an ''on'' position.
As MMP degradation of the ECM liberates the matrix-bound TIMP-3, it can then
inhibit any subsequent MMP activation at the cell-ECM level [ 100 ]. Normally,
TIMP-3 functions to form complexes via its C-domain with MMPs-2 and -9, thus
effectively slowing pro-angiogenic signaling [ 90 ].
TIMP-4 is found mainly in the human heart, with low levels of the inhibitor
found in the kidney, colon, placenta, and testes [ 101 ]. Levels are dysregulated in
various cardiovascular diseases. This TIMP functions mainly to reduce EC motility,
as well as proliferation, and induce apoptosis as well. In in vivo models, addition of
TIMP-2 results in suppression of angiogenesis, while addition of TIMP-4 is not able
to have this same effect [ 102 ]. TIMP-4 deficient mice instead showed reduced
cardiac function with aging, due to increased apoptosis of cells [ 103 ].
An additional inhibitor of MMPs, while not in the TIMP family, is the GPI-
anchored glycoprotein, reversion-inducing cysteine-rich protein with kazal motifs
(RECK). It is known to inhibit the release of pro-MMP-9 from the EC surface. It also
effectively inhibits MT1-MMP, which will result in inhibition of MMP-2, as
previously discussed [ 104 , 105 ]. In RECK knockout mice, blood vessels cannot
reach a mature stage, and mice die in utero. Overexpression of RECK in tumor
models results in a reduction of new blood vessel sprouting to sufficiently nourish
the tumor [ 88 ]. An additional proteinase inhibitor, a 2 -macroglobulin, is the primary
MMP inhibitor found in blood plasma [ 106 ]. Finally, thrombospondin-1, a known
inhibitor of angiogenesis, is also known to inhibit pro-MMP-2 and
pro-MMP-9 from becoming activated. Thrombospondin-2 is also known to complex
with MMP-2 to increase clearance via receptor-mediated endocytosis [ 106 ].
A schematic summarizing many of the effects of soluble and membrane-bound
MMPs, as well as the TIMPs, is shown in Fig. 5 .
4 Effects of Stromal Cells on the ECM
A. Stromal cells influence ECM synthesis and degradation
It has been widely established in the literature that the presence of pericytes
covering EC tubules results in stabilization of the vessels, a decrease in vascular
pruning, and decreased permeability of the nascent vessels [ 107 ]. Pericytes are a
source of angiopoietin-1, which acts on EC TIE-2 to stabilize these heterogeneous
cell-cell junctions [ 108 ]. Recent findings have shown that EC-pericyte interactions
occur after ECs carve out ''vascular guidance tunnels'' within the ECM, which