Parstatin Suppresses Ocular Neovascularization and Inflammation

Angiogenesis, the formation of new blood vessels, is a well-established and clinically relevant feature of a variety of disease states.1 For instance, a plethora of studies support the notion that the rate of tumor growth and resultant cancer progression is dependent on the establishment of new blood vessels to supply oxygen and nutrients to proliferating cancer cells.2 Additionally, aberrant neovascularization, remodeling, fibrosis, and inflammation occur in association with retinal and choroidal ischemic or degenerative diseases such as age-related macular degeneration, diabetic retinopathy, and retinopathy of prematurity3 and result from infection and mechanical or chemical injury to the cornea.4 Proteinase-activated receptor 1 (PAR1) is a G protein–coupled receptor that participates in hemostasis and vascular development5 and mediates the angiogenic activity of thrombin.6 PAR1, which is expressed on a variety of cell types including vascular endothelial and smooth muscle cells, is activated through proteolytic cleavage of the Arg41/Ser42 bond on its N-terminal extracellular domain by a variety of proteases, most notably thrombin.7 The new truncated N terminus serves as a tethered ligand that activates PAR1 itself. However, little is known about the role of the 41-amino acid peptide released during PAR1 activation. Recently, we have shown that this peptide, for which we coined the name parstatin, is a potent antiangiogenic factor.8 In several in vitro and ex vivo assay systems, including chick embryo chorioallantoic membrane assay, rat aortic ring, and endothelial cell tube formation, parstatin was able to suppress both fibroblast growth factor (FGF)-2– and vascular endothelial growth factor (VEGF)–mediated angiogenesis. Parstatin also suppressed endothelial cell growth by inhibiting the phosphorylation of extracellular signal-regulated kinases (Erk1/2, MAPK) in a specific and reversible fashion and by promoting cell cycle arrest and apoptosis through a mechanism involving the activation of caspases. Based on these findings and the fact that parstatin is an endogenously formed peptide with relative cell-type specificity and high efficacy, we hypothesized that this agent had therapeutic potential in the treatment of neovascular ocular diseases. In the present study, we evaluated the effect of parstatin in well-established models of ocular neovascularization and inflammation.