Proteases in angiogenesis
It is a highly complex process involving extensive interplay between cells, soluble factors, and the extracellular matrix (ECM).
Angiogenesis is critical during normal physiological development, but it also occurs in adults during inflammation, wound healing, ischemia, and in pathological conditions such as rheumatoid arthritis, hemangioma, and tumor growth.
The role of matrix metalloproteinases and TIMPs in several pathological conditions including angiogenesis, tumor growth, and metastasis has been investigated and very well described.
[3][4] ADAMs comprise a family of integral membrane as well as secreted glycoproteins which are related to snake venom metalloproteinases and MMPs.
[7] ADAMs participate in a wide variety of cell surface remodeling processes, including ectodomain shedding, regulation of growth factor availability and mediating cell-matrix interactions.
[8] ADAMTS are a subfamily of ADAM related metalloproteinases that contain at least one thrombospondin type I sequence repeat motif (TSR).
A highly characterized example of the serine protease family is the plasminogen activator-plasmin system, which has been shown to be involved in vascular remodelling .
The activated form of plasminogen, plasmin, is a wide-ranging protease capable of acting on various ECM components including fibrin, collagens, laminin, fibronectin, and proteoglycans.
Notch-1 signaling is essential for endothelial differentiation, and tumor angiogenesis, while the angiopoietin receptor Tie-1 facilitates embryonic blood vessel formation.
These findings indicate that ectodomain shedding is a ubiquitous process facilitating a wide variety of cellular events involved in angiogenesis.
The formation of capillaries from pre-existing blood vessels requires the remodeling of both the peicapillary membrane of the parent venule, as well as the local and distal ECM.
At the onset of angiogenesis endothelial cells (EC) must remodel three different barriers in order to migrate and invade the target tissue.
First is the basement membrane between the endothelium and vascular smooth muscle cells or pericytes, followed by the fibrin gel formed from fibrinogen that is leaked from the vasculature, and finally the extracellular matrix in the target tissue.
[19] After working their way through the basement membrane, EC must invade through a dense fibrin gel which is polymerized from fibrinogen derived from the vascular bed.
[20] Plasmin, an effective fibrinolysin produced by tPA or uPA, was thought to be essential in this process, but plasminogen deficient mice do not display major defects of neovascularization in fibrin rich tissues.
[23] Numerous proteolytic fragments or domains of ECM proteins have been reported to exert positive or negative activity on angiogenesis.
Nackman et al. demonstrated that elastase generated elastin fragments mediate several characteristic features of aneurysmal disease which correlated to angiogenesis.
Endogenous inhibitors or activators generated by proteolytic degradation of larger proteins mostly from the ECM have proven to contribute to the regulation of tumor growth and angiogenesis.
Proteases not only modulate cell-matrix interactions but also can control the onset and progression of angiogenesis by activating angiogenic growth factors and cytokines.
VEGF121 and the truncated VEGF165, in contrast, cause irregular patterns of neovascularization, most likely due to their inability to bind heparan sulfates, wherefore they do not provide any spatial information that is buried in the ECM.
[29] The ADAM family of proteases is receiving increased attention for their ability to alter the balance between pro-and anti-angiogenic factors.
ADAM17 is able to release active tumor necrosis factor-alpha (TNFα) and heparin-binding EGF-like growth factor (HB-EGF) from their membrane bound precursors which can indirectly affect angiogenesis.
A recombinant disintegrin domain of human ADAM15 inhibits a variety of EC functions in vitro including proliferation, adhesion, migration, and capillary formation.
Unexpectedly, when PAI-1 deficient mice were challenged with cancer cells on a collagenous matrix, angiogenesis and vascular stabilization was inhibited, hampering tumor growth.
Uncontrolled proteolysis also is attributed to the disruption of vascular development and premature deaths in murine embryos deficient of the inhibitor reversion-inducing-cysteine-rich protein with kazal motifs (RECK).
Cathepsin L1 is active at neutral pH by associating with a p41 splice variant of the MHC class II-associated invariant chain which is strongly expressed in EPCs.
[43] This ability to stay active at neutral pH may facilitate EPC invasion, remodeling of matrix collagens and gelatin, and neovascularization.
It has been well established that smooth muscle-like pericytes play an important role in stabilizing newly formed blood vessels.
Pericytes secrete TIMP-3 which inhibits MT1-MMP dependent MMP-2 activation on endothelial cell, thus facilitating stabilization of newly formed microvessels.
Perhaps more significantly, a new paradigm has emerged for proteases being essential for modulating growth factors and cytokines, generating biologically active fragments from the matrix, facilitating recruitment of bone marrow derived cells, and stabilization of mature blood vessels.
