Major reviewMatrix metalloproteinase 14 modulates signal transduction and angiogenesis in the cornea
Introduction
Neovascularization (NV) is the term used to describe the local formation of new vascular structures at previously avascular sites. Several models of neovascular processes have been proposed10, 14, 18, 24, 25, 43, 81, 104, 117 including 1) vasculogenesis, which is the formation of new blood vessels from bone marrow-derived angioblasts, predominantly during embryogenesis; 2) local recruitment of endothelial progenitor cells; and 3) angiogenesis, which is the formation of new vessels from preexisting vascular structures.31, 170 Angiogenesis is a common feature of corneal and retinal disorders as well as cancer metastasis and is usually stimulated by changes in the endothelial cell microenvironment (e.g., trauma, hypoxia, oxidative stress, mechanical strain, and genetic changes).46 Physical changes that occur during NV include extracellular matrix (ECM) degradation, ECM remodeling, cellular migration, and cellular invasion. The pathologies associated with NV in the normally avascular cornea include herpetic stromal keratitis, diverse inflammatory disorders, systemic or autoimmune diseases, corneal graft rejection, infectious keratitis (and other corneal infection/inflammation), contact lens–related hypoxia, alkali burns, stromal ulceration, corneal epithelium weakness, recurrent erosion syndrome, diabetes mellitus–related epithelial weakening, and limbal stem cell deficiency.171 Corneal NV is also seen in some congenital disorders such as aniridia, which involves complete or partial absence of the iris. In such conditions, the balance between proangiogenic and antiangiogenic factors favors NV, with both upregulation of proangiogenic factors, such as vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF), and downregulation of antiangiogenic factors, such as endostatin and thrombospondin-1.43, 47, 176
The cornea is avascular under normal conditions and, even when damaged, typically heals without NV.24 This persistent avascularity of the cornea, which is necessary for vision clarity, may be facilitated by: 1) the tightly organized packing of collagen fibrils, 2) the angiostatic nature of corneal epithelial cells, 3) the immune privilege of the cornea, mediated by factors such as transforming growth factor-β (TGF-β) in tears, 4) the comparatively hypothermic nature of the cornea, 5) extensive neuronal innervation, 6) the movement of the aqueous humor on the endothelium, 7) low levels of proangiogenic matrix metalloproteinases (MMPs), 8) active production of antiangiogenic factors after corneal injury, and 9) the barrier function of the limbus.14, 43
To maintain corneal clarity, proper corneal wound healing is critical. Corneal stromal wound healing occurs in 4 phases. In the first phase the keratocytes adjacent to the area of the epithelial defect undergo apoptosis, leaving a central zone devoid of cells. This cell death has been suggested to initiate the healing response.111 In the second phase the keratocytes immediately adjacent to the area of cell death proliferate to repopulate the wound area. For example, in rat corneas, proliferation occurs 24 to 48 hours postwound.34 Within this phase the keratocytes transform into fibroblasts that migrate into the wound area, a process which may take up to 1 week.34 This transformation is evident at the molecular level, with reorganization of the actin cytoskeleton in the development of stress fibers and focal adhesion structures and activation of new genes encoding ECM components, such as fibronectin, cell adhesion molecule, α5 integrin, ECM-degrading MMPs, and cytokines.14, 15, 16 The same transition can be seen in vitro. When keratocytes are isolated from the corneal stroma and subcultured in serum-containing medium, they acquire the fibroblast phenotype.104 The migratory repair fibroblasts contain filamentous-actin and are elongated, spindle shaped, and highly reflective. These fibroblasts induce the synthesis of the α5 integrin chain, which results in the formation of the α5β1 integrin heterodimer, the classic fibronectin receptor. This occurs concomitant with a reduction in fibronectin content in the wound area. In addition to forming the ECM, these repair fibroblasts synthesize several MMPs, including MMP-1, -2, -3, -9, and -14.104
In the third phase of stromal wound healing, transformation of fibroblasts into myofibroblasts may occur and can be observed via α-smooth muscle actin staining. Myofibroblasts appear as stellate cells and are highly reflective, but are limited to within the wound area. The extent of fibroblast transformation into myofibroblasts seems to be dependent on the type of wound and the integrity of the Bowman membrane. In general, gaping wounds and wounds in which the Bowman membrane is removed result in greater myofibroblast generation than wounds that do not penetrate the Bowman membrane. This process, which may take up to 1 month to become histologically apparent, can lead to a decrease in corneal clarity and vision deterioration.
The fourth and final phase of stromal wound healing involves stromal remodeling and is largely dependent on the characteristics of the original wound. Within this setting, intricate, but incompletely understood, relationships among keratocytes, fibroblasts, and myofibroblasts play key roles.45 It is theorized that TGF-β and fibroblast-like synoviocytes are key intermediaries in the process of remodeling.34 Wounds that have completely healed contain few, if any, myofibroblasts, presumably because these cells revert to the fibroblast phenotype or undergo apoptosis during wound healing.69 The entire process of corneal healing after an injury may take more than 1 year.34
In the setting of certain inflammatory, infectious, degenerative, and traumatic states, corneal NV may be induced during wound healing.24, 183 When NV does occur, blood vessel invasion into the cornea is associated with significant visual impairment, which can ultimately progress to blindness. Three distinct morphologies of corneal NV are most commonly diagnosed: 1) deep NV overlying Descemet membrane (Fig. 1A), 2) stromal NV observed in interstitial keratitis (Fig. 1B), and 3) superficial vascular pannus (Fig. 1C).
NV occurs when there is a disturbance in the balance between proangiogenic and antiangiogenic factors. Proangiogenic factors include VEGF, bFGF (also referred to as FGF-2), and platelet-derived growth factor (PDGF). Antiangiogenic factors include angiostatin, endostatin, pigment epithelium-derived factor (PEDF), thrombospondin-1, and soluble VEGF receptor 1 (VEGFR1). A striking indication of this delicate balancing act is that many of the antiangiogenic factors are proteolytic degradation products derived from ECM fragments formed during the initial invasion of cells into the ECM during angiogenesis.7, 14 MMPs have been implicated as both proangiogenic and antiangiogenic molecules that are, in part, responsible for orchestrating the delicate balance between corneal angiogenesis and avascularity.14, 63 In this review, we present evidence for the facilitative role of MMPs, specifically MMP-14, in corneal NV.
Section snippets
MMP members
Among the 25 MMPs identified to date, at least 16 have been found in the cornea, including collagenases (MMP-1, -8, and -13), gelatinases A and B (MMP-2 and -9), stromelysins (MMP-3, -10, and -11), matrilysin (MMP-7), macrophage metalloelastase (MMP-12), and the membrane type MMPs (MMP-14, -15, -16, -17, -24, -25).6, 9, 19, 24, 38, 39, 41, 65, 81, 98, 99, 114, 130, 134, 160, 168 MMPs display dual functions during angiogenesis based on their ability to 1) degrade ECM, which allows tissue
MMP-14 processing and regulation
The precise nature of MMP-14 processing, shedding, and endocytosis conveys unique regulatory qualities with inherent complexity. MMP-14 is produced as an inactive zymogen and then cleaved into a 57-kDa peptide that inserts into the plasma membrane. MMP-14 can undergo autocatalysis to generate a 44-kDa peptide that is an inactive degradation product. Osenkowski and colleagues demonstrated that the hinge region of MMP-14 is the “hot spot” for autocatalytic activity, which produces the 44-kDa
MMP-14 mechanisms of action
MMP-14 is the most prevalent MMP involved in angiogenesis and ECM remodeling.75, 120 It leads to the disruption of endothelial tight junctions, reorganization of the actin cytoskeleton, and proteolysis of the basement membrane and interstitial matrix. MMP-14 exerts its effects via 4 key mechanisms: 1) cleavage of ECM molecules, such as type I collagen,72, 156 2) upregulation of angiogenic factors, such as VEGF,156 3) interactions with the cellular adhesion molecules expressed on the cell
Pathways of MMP-14 signaling
MMP-14 expression is regulated through a multitude of signaling pathways, and its expression leads to the production of multiple factors that support or inhibit angiogenesis and ECM remodeling. Furthermore, MMP-14 expression is associated with various disease processes, such as NV and cancer metastasis, through its ability to degrade ECM components and participate in multiple signaling pathways. Such interactions illustrate the complexity of MMP-14 signaling and demonstrate the delicate balance
Conclusions
Corneal NV is a pathologic condition characterized by angiogenesis and ECM remodeling within the cornea, a physiologically avascular tissue. Proangiogenic factors such as MMP-14 play important roles in inducing corneal NV. The signaling pathways by which MMP-14 induces angiogenic processes and ECM degradation in conjunction with other proangiogenic factors such as VEGF, FGF, HGF, and Src have been summarized here.
Currently, anti-MMP14 (9EB and DX2400) antibodies have been successfully used to
Disclosure
No conflicts of interest, financial or otherwise, are declared by the authors.
Acknowledgments
This study was supported by grants from the National Institutes of Health EY10101 (Dimitri T. Azar), EY023691, EY021886, I01 BX002386 (Jin-Hong Chang), and EY01792, and an unrestricted grant from Research to Prevent Blindness, New York, NY.
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