Parstatin suppresses ocular neovascularization and inflammation.

PURPOSE: Parstatin is a 41-mer peptide formed by proteolytic cleavage on activation of the PAR1 receptor. The authors recently showed that parstatin is a potent inhibitor of angiogenesis. The purpose of the present study was to evaluate the therapeutic effect of parstatin on ocular neovascularization. METHODS: Choroidal neovascularization was generated in mice using laser-induced rupture of Bruch's membrane and was assessed after 14 days after perfusion of FITC-dextran. Oxygen-induced retinal neovascularization was established in neonatal mice by exposing them to 75% O(2) at postnatal day (P)7 for 5 days and then placing them in room air for 5 days. Evaluation was performed on P17 after staining with anti-mouse PECAM-1. The effect of parstatin was tested after intravitreal administration. The effects of subconjunctival-injected parstatin on corneal neovascularization and inflammation in rats were assessed 7 days after chemical burn-induced corneal neovascularization. Retinal leukostasis in mice was assessed after perfusion with FITC-conjugated concanavalin A. RESULTS: Parstatin potently inhibited choroidal neovascularization with an IC(50) of approximately 3 µg and a maximum inhibition of 59% at 10 µg. Parstatin suppressed retinal neovascularization with maximum inhibition of 60% at 3 µg. Ten-microgram and 30-µg doses appeared to be toxic to the neonatal retina. Subconjunctival parstatin inhibited corneal neovascularization, with 200 µg the most effective dose (59% inhibition). In addition, parstatin significantly inhibited corneal inflammation and VEGF-induced retinal leukostasis. In all models tested, scrambled parstatin was without any significant effect. CONCLUSIONS: Parstatin is a potent antiangiogenic agent of ocular neovascularization and may have clinical potential in the treatment of angiogenesis-related ocular disorders.

Parstatin, the cleaved peptide on proteinase-activated receptor 1 activation, is a potent inhibitor of angiogenesis.

The proteolytic activation by thrombin of the proteinase-activated receptor 1 unveils the tethered peptide ligand and cleaves a 41-amino acid peptide. In this report, we show that this peptide, which we have designated as "parstatin," is a potent inhibitor of angiogenesis. Synthesized parstatin suppressed both the basic angiogenesis and that stimulated by basic fibroblast growth factor and vascular endothelial growth factor in the chick embryo model in vivo and in the rat aortic ring assay. Parstatin also abrogated endothelial cell migration and capillary-like network formation on the Matrigel and fibrin angiogenesis models in vitro. Treatment of endothelial cells with parstatin resulted in inhibition of cell growth by inhibiting the phosphorylation of extracellular signal-regulated kinases in a specific and reversible fashion and by promoting cell cycle arrest and apoptosis through a mechanism involving activation of caspases. We have shown that parstatin acts as a cell-penetrating peptide, exerting its biological effects intracellularly. The uptake into cells and the inhibitory activity were dependent on parstatin hydrophobic region. These results support the notion that parstatin may represent an important negative regulator of angiogenesis with possible therapeutic applications.

Thrombin's central role in angiogenesis and pathophysiological processes.

A plethora of endogenous modulators of angiogenesis have been identified and their roles in the molecular and cellular events that mediate and regulate angiogenesis have been proposed. In this review, we summarize the recent findings on the role of thrombin/thrombosis on angiogenesis and other related pathophysiological processes. The mechanisms by which thrombin itself and its receptor PAR1 orchestrate many cellular events through interaction with a variety of other factors and cell types are discussed. These new data point to the complexity of the regulatory processes involved in the angiogenic cascade, which may be tissue specific, and dependent upon the pathology involved. The understanding of these events may provide targets for therapeutic intervention in disease states where angiogenesis is disturbed.

Thrombin mediates mitogenesis and survival of human endothelial cells through distinct mechanisms.

Thrombin has been reported to play a pivotal role in the initiation of angiogenesis by indirectly regulating and organizing a network of angiogenic molecules. In addition, it has been proposed that thrombin can directly activate endothelial cell proliferation. However, in this report it was shown that thrombin is a poor growth factor for human endothelial cells, and its modest mitogenic activity is mediated indirectly by the release of heparin-binding epidermal growth factor, subsequent to proteinase-activated receptor 1 (PAR1) activation. On the other hand, it was demonstrated that thrombin is a potent anti-apoptotic factor for endothelial cells, pointing to a novel role of thrombin in vascular protection. Analysis by annexin V-propidium iodide double staining revealed that thrombin, specifically, promoted survival of serum-starved endothelial cells in a concentration-dependent manner. In contrast to its mitogenic effect, the anti-apoptotic effect of thrombin was largely independent of its catalytic activity and was mediated through interaction with alphanubeta3 and alpha5beta1 integrins, whereas the involvement of PAR1 was limited. These results provide new insights in understanding the role of thrombin in endothelial cell signaling and vascular biology.

Inhibition of angiogenesis by small-molecule antagonists of protease-activated receptor-1.

Angiogenesis, the growth of new blood vessels from preexisting ones, is a necessary component of embryogenesis, wound healing, and the proliferative phase of the female reproductive cycle. Angiogenesis also plays a critical role in important pathologic processes such as cancer and cardiovascular complications. In addition, clinical, laboratory, and pharmacologic evidence has shown a link between angiogenesis, coagulation, hemostasis, and thrombosis in the settings of these pathologies. Recent studies in our laboratory revealed that thrombin has a significant stimulatory effect on angiogenesis. This effect of thrombin is independent of fibrin formation and can be attributed mainly to the activation of protease-activated receptor-1 (PAR-1). PAR-1 is widely expressed in vascular cells and is involved in cardiovascular complications such as atherosclerosis, restenosis, and neointimal formation. It is also expressed in many cancer cells contributing to induction of tumor growth and metastasis. In this review, we will summarize our present-day understanding of the role of thrombin and PAR-1 in angiogenesis and the potential therapeutic utility of targeting PAR-1 in angiogenesis-related disease, such as atherosclerosis, restenosis, and cancer.

Blockade of angiogenesis by small molecule antagonists to protease-activated receptor-1: association with endothelial cell growth suppression and induction of apoptosis.

Many studies support the notion that protease-activated receptor (PAR)-1 plays a pivotal role in angiogenesis. However, direct evidence and understanding the molecular mechanisms involved were limited because PAR-1-specific antagonists have been developed only recently. In the present study, we evaluated the effects of two well characterized PAR-1 antagonists, SCH79797 ((N-3-cyclopropyl-7-{[4-(1-methylethyl)phenyl]-methyl}-7H-pyrrolo[3,2-f]quinazoline-1,3-diamine)) and RWJ56110 [(alphaS)-N-[(1S)-3-amino-1-[[(phenylmethyl)amino]carbonyl]propyl]-alpha-[[[[[1-(2,6-dichlorophenyl)methyl]-3-(1-pyrrolidinylmethyl)-1H-indol-6-yl]amino]carbonyl]amino]-3,4-difluorobenzenepropanamide], in the angiogenic cascade. These antagonists suppressed both the basic angiogenesis and that stimulated by thrombin in the chick chorioallantoic membrane model in vivo. PAR-1 antagonists also abrogated tube formation in the in vitro Matrigel system. These inhibitory effects were dose-dependent and well correlated with the inhibitory effects of SCH79797 and RWJ56110 on primary endothelial cell proliferation and on the initiation of apoptosis. PAR-1 blockage resulted in inhibition of endothelial cell growth by increasing the sub-G0/G1 fraction and reducing the percentage of cells in the S phase. Consistent with this, PAR-1 antagonists reduced incorporation of [3H]thymidine in endothelial cells and blocked the phosphorylation of extracellular signal-regulated kinases in a fashion depending specifically on PAR-1 activation. Analysis by annexin V/propidium iodide staining and poly(ADP-ribose) polymerase cleavage revealed that PAR-1 blockage increased apoptotic cell death by a mechanism involving caspases. These results provide further evidence that PAR-1 is a key receptor that mediates angiogenesis and suggest PAR-1 as target for developing antiangiogenic agents with potential therapeutic application in cancer and other angiogenesis-related diseases.

Thrombin functions through its RGD sequence in a non-canonical conformation.

Previous studies have suggested that thrombin interacts with integrins in endothelial cells through its RGD (Arg-187, Gly-188, Asp-189) sequence. All existing crystal structures of thrombin show that most of this sequence is buried under the 220-loop and therefore interaction via RGD implies either partial unfolding of the enzyme or its proteolytic digestion. Here, we demonstrate that surface-absorbed thrombin promotes attachment and migration of endothelial cells through interaction with alpha(v)beta(3) and alpha(5)beta(1) integrins. Using site-directed mutants of thrombin we prove that this effect is mediated by the RGD sequence and does not require catalytic activity. The effect is abrogated when residues of the RGD sequence are mutated to Ala and is not observed with proteases like trypsin and tissue-type plasminogen activator, unless the RGD sequence is introduced at position 187-189. The potent inhibitor hirudin does not abrogate the effect, suggesting that thrombin functions through its RGD sequence in a non-canonical conformation. A 1.9-Angstroms resolution crystal structure of free thrombin grown in the presence of high salt (400 mm KCl) shows two molecules in the asymmetric unit, one of which assumes an unprecedented conformation with the autolysis loop shifted 20 Angstroms away from its canonical position, the 220-loop entirely disordered, and the RGD sequence exposed to the solvent.

On the mode of action of thrombin-induced angiogenesis: thrombin peptide, TP508, mediates effects in endothelial cells via alphavbeta3 integrin.

In a previous report we have presented evidence that thrombin interacts with alpha(v)beta(3) integrin in endothelial cells at the molecular and cellular level. This interaction was shown to be of functional significance in vitro and in vivo and contributed to activation of angiogenesis by thrombin. In the present study, we have used a synthetic thrombin peptide, TP508, which represents residues 183 to 200 of human thrombin. This peptide lacks the catalytic site of thrombin but contains the thrombin RGD sequence. Immobilized (surface-coated) TP508 peptide, like thrombin, supported alpha(v)beta(3) integrin-dependent endothelial cell attachment and haptotactic migration. These effects were specific (a scrambled TP508 peptide was without effect), and dosedependent. The RGD sequence was essential since a modified TP508 peptide, which contained RAD sequence instead of RGD, was inactive. Immobilized TP508 peptide stimulated phosphorylation of mitogen-activated protein kinases and focal adhesion kinase, the signal transduction pathways characteristic for integrin activation. On the other hand, TP508 peptide, when in solution, did not mimic other thrombin-promoted angiogenic effects, such as that of activation gelatinase A, upregulation of expression of vascular endothelial growth factor receptor mRNA or prostacyclin PGI(2) release in endothelial cells. On the contrary, soluble TP508 acted as an antagonist for the aforementioned effects of thrombin. TP508 peptide inhibited these thrombin-induced effects through a RGD and alpha(v)beta(3)-related mechanism. The antagonism with thrombin or thrombin receptor activating peptide was specific and involved at least in part mitogen-activated protein kinases activation. These results point to the importance of RGD sequence of thrombin in mediating effects on endothelial cells and angiogenesis.

Role of thrombin in angiogenesis and tumor progression.

Clinical, laboratory, histopathological, and pharmacological evidence support the notion that the coagulation system, which is activated in most cancer patients, plays an important role in tumor biology. Our laboratory has provided evidence that thrombin activates angiogenesis, a process which is essential in tumor growth and metastasis. This event is independent of fibrin formation. At the cellular level many actions of thrombin can contribute to activation of angiogenesis: (1). Thrombin decreases the ability of endothelial cells to attach to basement membrane proteins. (2). Thrombin greatly potentiates vascular endothelial growth factor- (VEGF-) induced endothelial cell proliferation. This potentiation is accompanied by up-regulation of the expression of VEGF receptors (kinase insert domain-containing receptor [KDR] and fms-like tyrosine kinase [Flt-1]). (3). Thrombin increases the mRNA and protein levels of alpha (v)beta (3) integrin and serves as a ligand to this receptor. Furthermore, thrombin increases the secretion of VEGF and enhances the expression and protein synthesis of matrix metalloprotease-9 and alpha (v)beta (3) integrin in human prostate cancer PC-3 cells. These results could explain the angiogenic and tumor-promoting effect of thrombin and provide the basis for development of thrombin receptor mimetics or antagonists for therapeutic application.

Novel complexes of guanylate cyclase with heat shock protein 90 and nitric oxide synthase.

Soluble guanylate cyclase (sGC) is an important downstream intracellular target of nitric oxide (NO) that is produced by endothelial NO synthase (eNOS) and inducible NO synthase (iNOS). In this study, we demonstrate that sGC exists in a complex with eNOS and heat shock protein 90 (HSP90) in aortic endothelial cells. In addition, we show that in aortic smooth muscle cells, sGC forms a complex with HSP90. Formation of the sGC/eNOS/HSP90 complex is increased in response to eNOS-activating agonists in a manner that depends on HSP90 activity. In vitro binding assays with glutathione S-transferase fusion proteins that contain the alpha- or beta-subunit of sGC show that the sGC beta-subunit interacts directly with HSP90 and indirectly with eNOS. Confocal immunofluorescent studies confirm the subcellular colocalization of sGC and HSP90 in both endothelial and smooth muscle cells. Complex formation of sGC with HSP90 facilitates responses to NO donors in cultured cells (cGMP accumulation) as well as in anesthetized rats (hypotension). These complexes likely function to stabilize sGC as well as to provide directed intracellular transfer of NO from NOS to sGC, thus preventing inactivation of NO by superoxide anion and formation of peroxynitrite, which is a toxic molecule that has been implicated in the pathology of several vascular diseases.

On the mechanism of thrombin-induced angiogenesis: involvement of alphavbeta3-integrin.

Thrombin has been reported to be a potent angiogenic factor both in vitro and in vivo, and many of the cellular effects of thrombin may contribute to activation of angiogenesis. In this report we show that thrombin-treatment of human endothelial cells increases mRNA and protein levels of alphavbeta3-integrin. This thrombin-mediated effect is specific, dose dependent, and requires the catalytic site of thrombin. In addition, thrombin interacts with alphavbeta3 as demonstrated by direct binding of alphavbeta3 protein to immobilized thrombin. This interaction of thrombin with alphavbeta3-integrin, which is an angiogenic marker in vascular tissue, is of functional significance. Immobilized thrombin promotes endothelial cells attachment, migration, and survival. Antibody to alphavbeta3 or a specific peptide antagonist to alphavbeta3 can abolish all these alphavbeta3-mediated effects. Furthermore, in the chick chorioallantoic membrane system, the antagonist peptide to alphavbeta3 diminishes both basal and the thrombin-induced angiogenesis. These results support the pivotal role of thrombin in activation of endothelial cells and angiogenesis and may be related to the clinical observation of neovascularization within thrombi.