Parstatin inhibits viability of human retinal pigment epithelium cells: an in vitro cytotoxicity study

OBJECTIVE

Parstatin, the N-terminal 41-amino-acid peptide cleaved by thrombin from protease-activated-receptor 1, was shown to be highly effective in preventing ocular angiogenesis and as such it has potential therapeutic applications in ocular neovascular diseases. In the frame of a safety program in preclinical development, we investigated whether parstatin exerts any cytotoxic effect in critical ocular cell populations.

MATERIALS AND METHODS

Human retinal pigment epithelium cell-19 line (ARPE-19) and the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) colorimetric assay were used to investigate parstatin's effect in cell cultures. Parstatin 24-41 was used as a control peptide which lacks the hydrophobic domain to demonstrate the specificity and the structure-dependent effect of parstatin. Both peptides were used at concentrations ranging from 0.1-30 μM for 24, 48 and 72 hours.

RESULTS

In the presence of parstatin the rate of ARPE-19 cell growth and viability were significantly decreased in a concentration-dependent and time-dependent manner, with an IC50 of approximately 10 μM. When ARPE-19 cells were treated with parstatin 24-41 no inhibitory effect was observed at any concentration or exposure time used.

CONCLUSIONS

Parstatin has a clear detrimental effect on the viability of ARPE-19 cells and raises concerns about its use in the eye because of its possible toxic effects.

On the mode of action of thrombin-induced angiogenesis: thrombin peptide, TP508, mediates effects in endothelial cells via αvβ3 integrin

In a previous report we have presented evidence that thrombin interacts with αvβ3 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 αvβ3 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 PGI2 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 α. vβ3-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.

Design, synthesis and biological evaluation of new peptide-based ureas and thioureas as potential antagonists of the thrombin receptor PAR1

By applying a diversity oriented synthesis strategy for the search of new antagonists of the thrombin receptor PAR1, a series of peptide-based ureas and thioureas, including analogues of the PAR1 reference antagonist RWJ-58259, has been designed and synthesized. The general synthetic scheme involves reduction of basic amino acid-derived amino nitriles by hydrogen transfer from hydrazine monohydrate in the presence of Raney Ni, followed by reaction with diverse isocyanates and isothiocyanates, and protecting group removal. All new compounds have been evaluated as inhibitors of human platelet aggregation induced by the PAR1 agonist SFLLRN. Some protected peptide-based ureas displayed significant antagonist activity.

The protease-activated receptor 1 possesses a functional and cleavable signal peptide which is necessary for receptor expression

The protease-activated receptor 1 (PAR1) is activated by thrombin cleavage releasing the physiologically-relevant parstatin peptide (residues 1-41). However, the actual length of parstatin was unclear since the receptor may also possess a cleavable signal peptide (residues 1-21) according to prediction programs. Here, we show that this putative signal peptide is indeed functional and removed from the PAR1 resolving the question of parstatin length. Moreover, we show that the sequence encoding the signal peptide may surprisingly play a role in stabilization of the PAR1 mRNA, a function which would be novel for a G protein-coupled receptor.

Parstatin prevents renal injury following ischemia/reperfusion and radiocontrast administration

BACKGROUND/AIMS

Parstatin is a 41-mer peptide formed by proteolytic cleavage on activation of the protease-activated receptor 1. Parstatin was recently found to be cardioprotective against myocardial ischemia/reperfusion (IR) injury. In the present study, it was hypothesized that parstatin would protect the kidneys in acute renal failure.

METHODS

We investigated the effects of parstatin on the renal dysfunction and injury caused either by renal IR injury or contrast-induced nephropathy (CIN) in two animal models. Renal IR injury was induced in rats by bilateral occlusion of renal arteries and veins for 45 min followed by 4 h of reperfusion, while CIN was induced in rabbits by intravenous injection of the radiocontrast medium Iopromide.

RESULTS

Treatment with parstatin 15 min before or immediately after renal ischemia attenuated the resulting renal dysfunction as demonstrated by the improved biochemical indicators (serum creatinine and fractional excretion of Na(+)) and scintigraphic analysis. The effect was dose depended and provided evidence for a more prominent protection of tubular than glomerulal function. Histopathological examination of the kidneys revealed severe renal damage, which was significantly suppressed by the parstatin. Similarly, administration of a single dose of parstatin before the induction of CIN significantly protected against the resulting renal dysfunction and histologically evidenced renal tubular injury.

CONCLUSION

These results suggest that parstatin is able to act as nephroprotective agent and may be useful in enhancing the tolerance of the kidney against renal injury associated with clinical conditions of acute renal failure. Further investigation on the mechanism underlying the nephroprotective properties of parstatin is deemed necessary.

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(1-26): the putative signal peptide of protease-activated receptor 1 confers potent protection from myocardial ischemia-reperfusion injury

Parstatin, the N-terminal 41-amino-acid peptide cleaved by thrombin from the protease-activated receptor 1, protects against rat myocardial ischemia and reperfusion injury. In this study, we determined that the parstatin fragment 1-26, the putative signal peptide of protease-activated receptor 1, contains the functional domain of parstatin. We assessed a synthesized parstatin(1-26) peptide in an in vivo rat model of myocardial regional ischemia-reperfusion injury (n = 6/group). Infarct size in control rat hearts was 58 +/- 1% area at risk. Parstatin(1-26) was able to reduce infarct size to 13 +/- 1% (P < 0.001) and 22 +/- 1% area at risk (P < 0.01) when given before or after reperfusion. The infarct-sparing effects of parstatin(1-26) were abolished by inhibition of G(i) proteins (pertussis toxin), phosphoinositide 3-kinase/Akt (wortmannin), nitric-oxide synthase (NOS; N(G)-monomethyl-l-arginine), soluble guanylyl cyclase [1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ)], and sarcolemmal and mitochondrial K(ATP) channels [glibenclamide, 5-hydroxydecanoic acid, and sodium (5-(2-(5-chloro-2-methoxybenzamido)ethyl)-2-methoxyphenylsulfonyl) (methylcarbamothioyl)amide (HMR 1098)]. Parstatin(1-26) cardioprotection was also abolished by atractyloside, a mitochondrial permeability transition pore (mPTP) opener. The inhibitors and opener alone had no effect on infarct size. Furthermore, preischemic treatment with parstatin(1-26) increased Akt and endothelial NOS phosphorylation at the time of reperfusion. After a 120-min reperfusion, parstatin(1-26) increased nitric oxide levels (12 +/- 0.4 to 17 +/- 0.9 mmol/g tissue) and cyclic GMP levels (87 +/- 21 to 395 +/- 36 pmol/g tissue). Parstatin(1-26) treatment either before or after ischemia results in an extremely efficacious protection against ischemia-reperfusion injury that depends on a G(i) protein-mediated pathway involving mPTP, the end effector of the preconditioning pathway. This suggests that parstatin(1-26) has a potential therapeutic role in the treatment of ischemia and reperfusion injury.

Parstatin: a cryptic peptide involved in cardioprotection after ischaemia and reperfusion injury

AIMS

Thrombin activates protease-activated receptor 1 by proteolytic cleavage of the N-terminus. Although much research has focused on the activated receptor, little is known about the 41-amino acid N-terminal peptide (parstatin). We hypothesized that parstatin would protect the heart against ischaemia-reperfusion injury.

METHODS AND RESULTS

We assessed the protective role of parstatin in an in vivo and in vitro rat model of myocardial ischaemia-reperfusion injury. Parstatin treatment before, during, and after ischaemia decreased infarct size by 26%, 23%, and 18%, respectively, in an in vivo model of ischaemia-reperfusion injury. Parstatin treatment immediately before ischaemia decreased infarct size by 65% and increased recovery in ventricular function by 23% in an in vitro model. We then assessed whether parstatin induced cardioprotection by activation of a Gi-protein-dependent pathway. Gi-protein inactivation by pertussis toxin completely abolished the cardioprotective effects. The cardioprotective effects were also abolished by inhibition of nitric oxide synthase (NOS), extracellular signal-regulated kinases 1/2 (ERK1/2), p38 mitogen-activated protein kinase (p38 MAPK), and K(ATP) channels in vitro. Furthermore, parstatin increased coronary flow and decreased perfusion pressure in the isolated heart. The vasodilatory properties of parstatin were confirmed in rat coronary arterioles.

CONCLUSION

A single treatment of parstatin administered prior to ischaemia confers immediate cardioprotection by recruiting the Gi-protein activation pathway including p38 MAPK, ERK1/2, NOS, and K(ATP) channels. Parstatin exerts effects on both the cardiomyocytes and the coronary circulation to induce cardioprotection. This suggests a potential therapeutic role of parstatin in the treatment of cardiac injury resulting from ischaemia and reperfusion.

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 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.

Methylene blue inhibits angiogenesis in chick chorioallontoic membrane through a nitric oxide-independent mechanism

Angiogenesis is the process of generating new blood vessels from preexisting vessels and is considered essential in many pathological conditions. The purpose of the present study was to evaluate the effect of methylene blue in chick chorioallantoic membrane angiogenesis model in vivo. In this well characterized model, methylene blue inhibited angiogenesis in a concentration-dependent manner. In addition, when methylene blue was combined with sodium nitroprusside, a spontaneous generator of nitric oxide, an inhibition of angiogenesis was evident which was comparable with that observed by the application of methylene blue alone. Sodium nitroprusside, alone, caused a significant inhibition in basal angiogenesis. These results provide evidence that methylene blue inhibits angiogenesis independently of nitric oxide pathway and suggest that methylene blue may be useful for treating angiogenesis-dependent human diseases.

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.

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.

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.

On the mechanism(s) of thrombin induced angiogenesis

Promotion of tumour progression by thrombin is suggested by several clinical and laboratory observations. A plausible explanation for this effect of thrombin may be related to our previous findings that thrombin is a potent promoter of angiogenesis in the chick chorioallantoic membrane system (CAM) and in the Matrigel system in vivo. In this report we summarise the cellular and molecular actions of thrombin that could be contributing to the activation of angiogenic cascade. Treatment of endothelial cells with thrombin leads to activation of gelatinase A, which may allow for local dissolution of basement membrane, an essential first step of angiogenesis. Similarly thrombin-treated endothelial cells have diminished ability to adhere to collagen type IV and laminin. This new phenotype of endothelial cells can migrate and survive without attachment to extracellular matrix. Thrombin-treatment of endothelial cells increases the vectorial secretion of extracellular matrix proteins, a process essential at the final steps of angiogenesis. In addition, thrombin potentiates the VEGF-induced mitogenesis of endothelial cells. This can be explained by the upregulation of the VEGF receptors (KDR & flt-1) by thrombin treatment. All the aforementioned effects of thrombin are receptor mediated, dose-dependent and require only brief exposure of endothelial cells to thrombin for these actions of thrombin. The transduction mechanisms involved are via protein kinase C (PKC) and MAP-kinase pathways.

Effects of thrombin/thrombosis in angiogenesis and tumour progression

Laboratory, histopathological, pharmacological and clinical evidence support the notion that a systemic activation of blood coagulation is often present in cancer patients. On the other hand, epidemiological studies provide evidence of an increased risk of cancer diagnosis following primary thromboembolism. Moreover, the metastatic ability of human breast cancer cells is correlated with the number of thrombin receptors of these cells, and thrombin treatment of B16 melanoma cells dramatically increases the number of lung metastases in rats. We have proposed that these tumour-promoting effects of thrombin can be explained by the ability of thrombin to activate angiogenesis, an essential requirement for tumour progression. Many of the cellular events involved in the angiogenic cascade can be activated by thrombin. At the molecular level, brief exposure of endothelial cells to thrombin causes an upregulation of the receptors (KDR and Flt-1) of VEGF, the key angiogenic mediator. This results in a synergistic effect of thrombin and VEGF in the activation of angiogenesis. In addition, thrombin activates cancer cells to secrete VEGF, thus causing a mutual stimulation between EC and CA cells. Cancer cells exposed to thrombin secrete metalloproteinase 92 KD and overexpress the integrin a(v)b(3), all of which are involved in tumour metastasis.

On the mechanism of thrombin-induced angiogenesis. Potentiation of vascular endothelial growth factor activity on endothelial cells by up-regulation of its receptors

Many of the cellular actions of thrombin may contribute to the angiogenesis-promoting effect of thrombin reported previously. In this study, we investigated the interaction between thrombin and vascular endothelial growth factor (VEGF), the specific endothelial cell mitogen and key angiogenic factor. Exposure of human umbilical vein endothelial cells to thrombin sensitizes these cells to the mitogenic activity of VEGF. This thrombin-mediated effect is specific, dose-dependent and requires the activated thrombin receptor. Quantitative reverse transcription- polymerase chain reaction analysis reveals a time- and dose-dependent up-regulation of mRNA for VEGF receptors (KDR and flt-1). Optimal thrombin concentration for maximal expression of mRNA for KDR is 1.5 IU/ml (170% over controls) and appears 8-12 h after thrombin stimulation. Nuclear run-on experiments demonstrate that the up-regulation of KDR mRNA by thrombin occurred at the transcriptional level. In addition, functional protein of KDR receptor is increased to about 200% over control after 12 h of thrombin treatment. The up-regulation of KDR and flt-1 mRNA is also mimicked by the thrombin receptor activating peptide. These findings could explain at least in part the potent angiogenic action of thrombin.

On the mechanism of thrombin-induced angiogenesis: inhibition of attachment of endothelial cells on basement membrane components

Human umbilical vein endothelial cells (HUVECs) placed on plastic plates coated with collagen type IV or laminin adhered within 60 min to an extent of about 32 and 39%, respectively. Brief exposure of HUVECs to thrombin caused a marked dose-dependent inhibition of adhesion. Thrombin at 1 IU/ml caused 50% inhibition even after 5 min of exposure of HUVECs. This effect was reversible since reincubation of thrombin-treated HUVECs with fresh growth medium for 15 min restored their ability for attachment. This short-term inhibitory effect of thrombin on the adhesion of HUVECs to extracellular matrix components was specific and depended on the activation of thrombin receptor. Hirudin abolished this effect of thrombin. Similarly, the proteolytically inactive PPACK-thrombin had no effect, but when used in combination with thrombin prevents the inhibitory effect of thrombin. In addition, the thrombin receptor agonist peptide (TRAP) mimicked the effect of thrombin on HUVEC adhesion. The transduction mechanism involved in this action of thrombin seems to be via cAMP, since forskolin or the phosphodiesterase inhibitor 3-isobutyl-1-methyl-xanthine restored the ability of HUVECs that had been exposed to thrombin to adhere. This novel cellular action of thrombin on endothelial cells may represent an important early event in activation of the normally quiescent endothelial cells and initiation of the angiogenic cascade.

Putrescine: a novel inhibitor of glycosylation-induced cross-links in laminin

OBJECTIVE

To determine whether putrescine, a naturally occurring polyamine, is able to prevent nonenzymatic glycosylation-induced cross-linking of basement membrane components. Cross-linking, leading to the formation of advanced glycosylation end products (AGEs) has been proposed as a major mechanism contributing to structural and functional changes of the vascular wall, thus leading to microangiopathy.

METHODS

Laminin, a major basement membrane glycoprotein present in the microvasculature, was isolated and incubated in the presence of high glucose concentrations, in order to generate an in vitro diabetic environment. Putrescine was either present or absent from the incubation mixture. Formation of cross-links was assessed by two independent methods: gel electrophoresis and development of characteristic fluorescence.

RESULTS

Putrescine inhibited in a dose-dependent manner the formation of slower mobility bands in gel electrophoresis. Quantitation of the development of fluorescence characteristic of glycosylation-induced cross-links demonstrated that putrescine inhibited the formation of fluorescent products.

CONCLUSIONS

Putrescine may be a promising compound for inhibition of protein cross-linking and, therefore, could be used in the prevention of diabetic microangiopathy.

Validation of collagenous protein synthesis as an index for angiogenesis with the use of morphological methods

A method providing a biochemical index for the evaluation of promoters or inhibitors of angiogenesis in the chick chorioallantoic membrane (CAM) is here described and validated. This method is based on the determination of collagenous protein synthesis which takes place during new vessel formation. Validation was done by comparing collagenous protein synthesis to morphological methods of determining vascular density either by counting the number of vessels intersecting three concentric rings or by computer-assisted image analysis. Five compounds which promote or inhibit angiogenesis in the CAM were used for this purpose. The protein kinase C activator 4-beta-phorbol 12-myristate 13-acetate and alpha-thrombin increased collagenous protein synthesis and these results correlated with those obtained by using the two morphological methods mentioned above. Similarly, the inhibitors of angiogenesis, Ro318220, tricyclodecan-9-yl xanthate (D609), and 8,9-dihydroxy-7-methyl-benzo[b]quinolizinium bromide (GPA1734), reduced collagenous protein synthesis and vascular density (determined by image analysis or by counting the number of vessels intersecting three concentric rings) to a comparable degree. These results indicate that collagenous protein synthesis can be used as a reliable, reproducible, and unbiased index of angiogenesis in the chick chorioallantoic membrane.

Opposing effects on modulation of angiogenesis by protein kinase C and cAMP-mediated pathways

The role of cAMP-mediated pathway in modulating angiogenesis was investigated. We have shown previously that activators of protein kinase C (PKC) caused a marked increase in angiogenesis, while the specific inhibitor of PKC, Ro 318220 suppressed angiogenesis. Here we show that forskolin, which activates adenylate cyclase and elevates the intracellular levels of cAMP, and the Sp-diastereomer of adenosine cyclic-3',5'-monophosphothioate (Sp-cAMPS), caused a dose-dependent suppression of collagenous protein biosynthesis and angiogenesis in the chick chorioallantoic membrane system (CAM). The opposite modulation of angiogenesis by activators of PKC and elevated cAMP levels was further confirmed by the suppression of 4 beta-phorbol-12-myristate-13-acetate (4 beta-PMA)-stimulated angiogenesis by either forskolin or Sp-cAMPS. On the contrary, the Rp-diastereomer of adenosine cyclic-3',5'-monophosphothioate (Rp-cAMPS), which antagonises endogenous cAMP biochemical actions, had no effect on angiogenesis alone and did not suppress 4 beta-PMA stimulated angiogenesis. However, Rp-cAMPS antagonised the effect of forskolin and Sp-cAMPS on 4 beta-PMA induced angiogenesis. Similar results were obtained in the human umbilical vein endothelial cell tube formation assay. In this system, the PKC inhibitor, Ro 318220, caused a dose-dependent inhibition of 4 beta-PMA reversed this effect. Also, forskolin and Sp-cAMPS caused an inhibition in tube formation. These results indicate that increased levels of intracellular cAMP have a negative effect in normal angiogenesis and cause a large reduction of the promotion of angiogenesis resulting from PKC activation.

Leukotrienes C4 and D4 promote angiogenesis via a receptor-mediated interaction

The involvement of leukotrienes in angiogenesis was investigated in the in vivo chick chorioallantoic membrane system. In this system leukotrienes C4 and D4 promoted angiogenesis in a dose-dependent manner. Leukotriene B4 was ineffective. The potent and selective peptidyl leukotriene receptor antagonist, SK&F 104353-Z2, abolished the angiogenic effects of leukotrienes C4 and D4 and reduced unstimulated angiogenesis. These results indicate that leukotrienes C4 and D4 promote angiogenesis in the chick chorioallantoic membrane via a receptor-mediated interaction.

Protein kinase C involvement in the regulation of angiogenesis

Using the chick chorioallantoic membrane as a model system for the study of angiogenesis, we have shown that promoters of protein kinase C (PKC) such as 4-beta-phorbol-12-myristate-13-acetate (4-beta-PMA) and 1,2-dioctanoyl-sn-glycerol (DiC8) stimulated angiogenesis. This effect was specific since 4-alpha-PMA and 1,2-dioleoyl-sn-glycerol, which either do not activate or cannot reach PKC, were devoid of angiogenic activity. Furthermore, Ro 31-8220, a specific inhibitor of PKC, suppressed both basal and 4-beta-PMA- or DiC8-induced angiogenesis. Similar results were obtained with the commonly used inhibitor of PKC, 1-(5-isoquinoline-sulfonyl)-2-methylpiperazine and with tricyclodecan-9-yl-xanthogenate, an antitumor agent which has been suggested to be an inhibitor of PKC. Activation of PKC may be, therefore, an important signalling pathway in the initiation and control of the angiogenic response.

Thrombin promotes angiogenesis by a mechanism independent of fibrin formation

The role of thrombin in angiogenesis was investigated in the chick chorioallantoic membrane (CAM) system. alpha-Thrombin promoted angiogenesis in a dose-dependent fashion and at 8.4 pmol/disk reached a maximum of 78% above the control. At a higher dose of alpha-thrombin (25 pmol/disk) the angiogenic effect declines and this can be explained by desensitization of the thrombin receptor. The promotion of angiogenesis by alpha-thrombin is specific as evidenced by the reversal of this effect by hirudin, which binds both the catalytic and the anion-binding exosite of thrombin or by heparin, which binds thrombin and accelerates its inactivation by antithrombin III. gamma-Thrombin, which is catalytically active but lacks the anion-binding exosite required for clotting activity, promotes angiogenesis in the CAM in the same fashion and to the same extent as alpha-thrombin, at doses up to 130 pmol/disk. Phenylalanyl-propyl-arginine chloromethyl ketone (P-PACK)-thrombin, the catalytically inactive analogue of alpha-thrombin that retains the anion-binding exosite, had no significant effect on angiogenesis in the CAM. When combined with alpha-thrombin, P-PACK-thrombin abolished the angiogenesis-promoting effect of alpha-thrombin. These results suggest that alpha-thrombin can promote angiogenesis in the CAM through interaction with its catalytic site without the requirement for fibrin formation.