Intravitreal injection of plasminogen kringle 5, an endogenous angiogenic inhibitor, arrests retinal neovascularization in rats

The progress of assessment methods and treatments of neovascular glaucoma secondary to central retinal vein occlusion

Neovascular glaucoma is a condition that results from central retinal vein occlusion and often leads to blindness. Accurate evaluation and appropriate treatment are crucial for patients. However, there is currently no uniform and clear standard to differentiate between ischemic and non-ischemic central retinal vein occlusion. Also, the assessment of neovascular glaucoma progression is uncertain. Meanwhile, although pan-retinal photocoagulation is a standard treatment to prevent the onset of neovascular glaucoma, its actual efficacy and the timing of intervention remain highly controversial. It is still challenging to balance the risks of side effects in the visual field against the uncertain effectiveness of the treatment. This paper delves into the pathogenesis of neovascular glaucoma to understand the development of therapeutic approaches. By taking into account various assessment criteria of central retinal vein occlusion and neovascular glaucoma over the years, combining functional tests and morphological tests provides the most accurate and rigorous solution. The age of patients, the extent, location, and duration of retinal ischemia are the primary factors that affect the severity and extent of ischemic central retinal vein occlusion and induce serious complications. From the perspective of prevention and treatment, the ischemic index is closely related to the development of neovascularization. The paper provides essential insights into the mechanism, efficacy, complications, and optimal timing of pan-retinal photocoagulation. Comparing the treatment effects of pan-retinal photocoagulation and intravitreal anti-VEGF injections, we suggest a combination of both treatments to explore effective treatment with fewer side effects in the long term. This article details the debate on the above issues and explores ideas for the clinical diagnosis and preventive treatment of neovascular glaucoma that results from ischemic central retinal vein occlusion.

Kallistatin Deficiency Induces the Oxidative Stress-Related Epithelial-Mesenchymal Transition of Retinal Pigment Epithelial Cells: A Novel Protagonist in Age-Related Macular Degeneration

Purpose

Retinal pigment epithelium (RPE) dysfunction induced by oxidative stress-related epithelial-mesenchymal transition (EMT) of RPE is the primary underlying mechanism of age-related macular degeneration (AMD). Kallistatin (KAL) is a secreted protein with an antioxidative stress effect. However, the relationship between KAL and EMT in RPE has not been determined. Therefore we aimed to explore the impact and mechanism of KAL in oxidative stress-induced EMT of RPE.

Methods

Sodium iodate (SI) was injected intraperitoneally to construct the AMD rat model and investigate the changes in RPE morphology and KAL expression. KAL knockout rats and KAL transgenic mice were used to explain the effects of KAL on EMT and oxidative stress. In addition, Snail overexpressed adenovirus and si-RNA transfected ARPE19 cells to verify the involvement of Snail in mediating KAL-suppressed EMT of RPE.

Results

AMD rats induced by SI expressed less KAL in the retina, and KAL knockout rats showed RPE dysfunction spontaneously where EMT and reactive oxygen species (ROS) production increased in RPE. In contrast, KAL overexpression attenuated EMT and ROS levels in RPE, even in TGF-β treatment. Mechanistically, Snail reversed the beneficial effect of KAL on EMT and ROS reduction. Moreover, KAL ameliorated SI-induced AMD-like pathological changes.

Conclusions

Our findings demonstrated that KAL inhibits oxidative stress-induced EMT by downregulating the transcription factor Snail. Herein, KAL knockout rats may be an appropriate animal model for observing spontaneous RPE dysfunction for AMD-like retinopathy, and KAL may represent a novel therapeutic target for treating dry AMD.

Vascular Endothelial Growth Factor Receptor 1 Targeting Fusion Polypeptides with Stimuli-Responsiveness for Anti-angiogenesis

Genetically engineered fusion polypeptides have been investigated to introduce unique bio-functionality and improve some therapeutic activity for anti-angiogenesis. We report herein that stimuli-responsive, vascular endothelial growth factor receptor 1 (VEGFR1) targeting fusion polypeptides composed of a VEGFR1 (fms-like tyrosine kinase-1 (Flt1)) antagonist, an anti-Flt1 peptide, and a thermally responsive elastin-based polypeptide (EBP) were rationally designed at the genetic level, biosynthesized, and purified by inverse transition cycling to develop potential anti-angiogenic fusion polypeptides to treat neovascular diseases. A series of hydrophilic EBPs with different block lengths were fused with an anti-Flt1 peptide, forming anti-Flt1-EBPs, and the effect of EBP block length on their physicochemical properties was examined. While the anti-Flt1 peptide decreased phase-transition temperatures of anti-Flt1-EBPs, compared with EBP blocks, anti-Flt1-EBPs were soluble under physiological conditions. The anti-Flt1-EBPs dose dependently inhibited the binding of VEGFR1 against vascular endothelial growth factor (VEGF) as well as tube-like network formation of human umbilical vein endothelial cells under VEGF-triggered angiogenesis in vitro because of the specific binding between anti-Flt1-EBPs and VEGFR1. Furthermore, the anti-Flt1-EBPs suppressed laser-induced choroidal neovascularization in a wet age-related macular degeneration mouse model in vivo. Our results indicate that anti-Flt1-EBPs as VEGFR1-targeting fusion polypeptides have great potential for efficacious anti-angiogenesis to treat retinal-, corneal-, and choroidal neovascularization.

Sustained therapeutic effect of an anti-inflammatory peptide encapsulated in nanoparticles on ocular vascular leakage in diabetic retinopathy

Pigment epithelium-derived factor (PEDF), an endogenous Wnt signaling inhibitor in the serine proteinase inhibitors (SERPIN) super family, is present in multiple organs, including the vitreous. Significantly low levels of PEDF in the vitreous are found to associate with pathological retinal vascular leakage and inflammation in diabetic retinopathy (DR). Intravitreal delivery of PEDF represents a promising therapeutic approach for DR. However, PEDF has a short half-life after intravitreal injection, which represents a major hurdle for the long-term treatment. Here we report the prolonged therapeutic effects of a 34-mer peptide of the PEDF N-terminus, encapsulated in poly (lactic-co-glycolic acid) (PLGA) nanoparticles (PEDF34-NP), on DR. PEDF34-NP inhibited hypoxia-induced expression of vascular endothelial growth factor and reduced levels of intercellular adhesion molecule 1 (ICAM-1) in cultured retinal cells. In addition, PEDF34-NP significantly ameliorated ischemia-induced retinal neovascularization in the oxygen-induced retinopathy rat model, and significantly reduced retinal vascular leakage and inflammation in streptozotocin-induced diabetic rats up to 4 weeks after intravitreal injection, as compared to PLGA-NP control. Intravitreal injection of PEDF34-NP did not display any detectable toxicities to retinal structure and function. Our findings suggest that PEDF34-NP can confer sustained therapeutic effects on retinal inflammation and vascular leakage, having considerable potential to provide long-term treatment options for DR.

Effective treatment of intractable diseases using nanoparticles to interfere with vascular supply and angiogenic process

Angiogenesis is a vital biological process involving blood vessels forming from pre-existing vascular systems. This process contributes to various physiological activities, including embryonic development, hair growth, ovulation, menstruation, and the repair and regeneration of damaged tissue. On the other hand, it is essential in treating a wide range of pathological diseases, such as cardiovascular and ischemic diseases, rheumatoid arthritis, malignancies, ophthalmic and retinal diseases, and other chronic conditions. These diseases and disorders are frequently treated by regulating angiogenesis by utilizing a variety of pro-angiogenic or anti-angiogenic agents or molecules by stimulating or suppressing this complicated process, respectively. Nevertheless, many traditional angiogenic therapy techniques suffer from a lack of ability to achieve the intended therapeutic impact because of various constraints. These disadvantages include limited bioavailability, drug resistance, fast elimination, increased price, nonspecificity, and adverse effects. As a result, it is an excellent time for developing various pro- and anti-angiogenic substances that might circumvent the abovementioned restrictions, followed by their efficient use in treating disorders associated with angiogenesis. In recent years, significant progress has been made in different fields of medicine and biology, including therapeutic angiogenesis. Around the world, a multitude of research groups investigated several inorganic or organic nanoparticles (NPs) that had the potential to effectively modify the angiogenesis processes by either enhancing or suppressing the process. Many studies into the processes behind NP-mediated angiogenesis are well described. In this article, we also cover the application of NPs to encourage tissue vascularization as well as their angiogenic and anti-angiogenic effects in the treatment of several disorders, including bone regeneration, peripheral vascular disease, diabetic retinopathy, ischemic stroke, rheumatoid arthritis, post-ischemic cardiovascular injury, age-related macular degeneration, diabetic retinopathy, gene delivery-based angiogenic therapy, protein delivery-based angiogenic therapy, stem cell angiogenic therapy, and diabetic retinopathy, cancer that may benefit from the behavior of the nanostructures in the vascular system throughout the body. In addition, the accompanying difficulties and potential future applications of NPs in treating angiogenesis-related diseases and antiangiogenic therapies are discussed.

PRODUCTION AND APPLICATION OF ANGIOSTATINS FOR THE TREATMENT OF OCULAR NEOVASCULAR DISEASES

Angiostatins comprise a group of kringle-containing proteolytically-derived fragments of plasminogen/plasmin, which act as potent inhibitory mediators of endothelial cells proliferation and migration. Angiostatins are involved in modulation of vessel growth in healthy tissues and various pathological conditions associated with aberrant neovascularization. The aim of the present paper was to summarize available information, including our own experimental data, on prospects of angiostatin application for treatment of ocular neovascular diseases (OND), focusing on retinal pathologies and corneal injury. In particular, literature data on prospective and retrospective studies, clinical trials and animal models relating to the pathophysiology, investigation and management of OND are described. Special emphasis was made on the laboratory approaches of production of different angiostatin isoforms, as well as comparison of antiangiogenic capacities of native and recombinant angiostatin polypeptides. Several studies reported that angiostatins may completely abolish pathologic angiogenesis in diabetic proliferative retinopathy without affecting normal retinal vessel development and without exhibiting adverse side effects. Angiostatins have been tested as a tool for corneal antiangiogenesis target therapy in order to manage diverse ocular surface pathological conditions induced by traumas, chemical burns, previous surgery, chronic contact lens wear, autoimmune diseases, keratitis and viral infections (herpes, COVID-19), corneal graft rejection, etc. Among all known angiostatin species, isolated K5 plasminogen fragment was shown to display the most potent inhibitory activity against proliferation of endothelial cells via triggering multiple signaling pathways, which lead to cell death and resulting angiogenesis suppression. Application of adenoviral genetic construct encoding angiostatin K5 as a promising tool for OND treatment illustrates a vivid example of upcoming revolution in local gene therapy. Further comprehensive studies are necessary to elucidate the clinical potential and optimal regimes of angiostatinbased intervention modalities for treating ocular neovascularization.

Insulin-Like Growth Factor Binding Protein-Related Protein 1 Inhibit Retinal Neovascularization in the Mouse Model of Oxygen-Induced Retinopathy

PURPOSE

To explore the inhibitory effect of insulin-like growth factor binding protein-related protein 1 (IGFBP-rP1) on retinal angiogenesis and its underlying molecular mechanisms in the mouse model of oxygen-induced retinopathy (OIR).

METHODS

C57BL/6J mice were classified into three groups as control group, OIR nonintervention group, and OIR intervention group. Postnatal day 12 (P12) mice in OIR intervention group were received recombinant mouse IGFBP-rP1 (50, 100, and 200 ng/mL) intravitreal injection. Five days later, the proliferative neovascular responses were estimated by quantifying the new vessel areas in flattening retinal tissues stained by high molecular fluorescein isothiocyanate-dextran and counting the numbers of neovascular cell nuclei breaking through the internal limiting membrane in cross sections. Expressions of phospho-extracellular signal-regulated kinase 1/2 (p-ERK1/2), ERK1/2, and vascular endothelial growth factor (VEGF) proteins in retinal tissues were assessed by western blot analysis.

RESULTS

Irregular neovascularization, nonperfusion region, and fluorescence leakage were observed in OIR models. The expression of retinal p-ERK1/2 and VEGF proteins were significantly upregulated in OIR nonintervention group compared with control group. The area ratio of retinal new vessels and the number of neovascular cell nuclei in OIR intervention group both decreased significantly, following the downregulation of retinal p-ERK1/2 protein expression and VEGF protein expression in a dose-dependent manner. Moreover, there was no significant difference in retinal ERK1/2 protein expression.

CONCLUSIONS

IGFBP-rP1 inhibits retinal angiogenesis by blocking ERK signaling pathway and downregulating VEGF expression in the mouse model of OIR. It highlights the potential importance of IGFBP-rP1 serving as a target of gene therapy for retinal neovascularization in the future.

Voltage-Dependent Anion Channel-1, a Possible Ligand of Plasminogen Kringle 5

Kringle 5, the fifth fragment of plasminogen, is known to be important for inhibiting the proliferation and migration of vascular endothelial cell (VEC), while not having any effects on normal endothelial cells. Therefore, it may be a potential tumor therapy candidate. However, the ligand of the Kringle 5 in VEC has not yet been identified. In this study, the possible ligand of Kringle 5 in vitro was screened and validated using Ph.D.-7 phage display peptide library with molecular docking, along with surface plasma resonance (SPR). After four rounds of panning, the specific clones of Kringle 5 were confirmed using enzyme-linked immunosorbent assay (ELISA). The gene sequence analysis showed that they expressed the common amino sequence IGNSNTL. Then, using a NCBI BLAST, 103 matching sequences were found. Following the molecular docking evaluation and considering the acting function and pathway of the plasminogen Kringle 5 in the human body, the most promising candidate was determined to be voltage-dependent anion channel-1 (VDAC-1), which was able to bind to Kringle 5 at -822.65 J·mol-1 of the binding energy at the residues of Lys12, Thr19, Ser57, Thr188, Arg139, Asn214, Ser240 and Lys274. A strong dose-dependent interaction occurred between the VDAC-1 and Kringle 5 (binding constant 2.43 × 103 L·mol-1) in SPR observation. Therefore, this study proposed that VDAC-1 was a potential ligand of plasminogen Kringle 5, and also demonstrated that the screening and validation of protein ligand using phage display peptide library with the molecular docking, along with SPR, was a practicable application.

The role of pharmacogenetics and advances in gene therapy in the treatment of diabetic retinopathy

Diabetic retinopathy (DR) and its complications such as diabetic macular edema continue to remain a major cause for legal blindness in the developed world. While the introduction of anti-tVEGF agents has significantly improved visual outcomes of patients with DR, unpredictable response, largely due to genetic polymorphisms, appears to be a challenge with this therapy. With advances in identification of various genetic biomarkers, novel therapeutic strategies consisting of gene transfer are being developed and tested for patients with DR. Application of pharmacogenetic principles appears to be a promising futuristic strategy to attenuate diabetes-mediated retinal vasculopathy. In this comprehensive review, data from recent studies in the field of pharmacogenomics for the treatment of DR have been provided.

Delivery strategies for treatment of age-related ocular diseases: From a biological understanding to biomaterial solutions

Age-related ocular diseases, such as age-related macular degeneration (AMD), diabetic retinopathy, and glaucoma, result in life-long functional deficits and enormous global health care costs. As the worldwide population ages, vision loss has become a major concern for both economic and human health reasons. Due to recent research into biomaterials and nanotechnology major advances have been gained in the field of ocular delivery. This review provides a summary and discussion of the most recent strategies employed for the delivery of both drugs and cells to the eye to treat a variety of age-related diseases. It emphasizes the current challenges and limitations to ocular delivery and how the use of innovative materials can overcome these issues and ultimately provide treatment for age-related degeneration and regeneration of lost tissues. This review also provides critical considerations and an outlook for future studies in the field of ophthalmic delivery.

The Small Molecule R-(-)-β-O-Methylsynephrine Binds to Nucleoporin 153 kDa and Inhibits Angiogenesis

R-(-)-β-O-methylsynephrine (OMe-Syn) is a naturally occurring small molecule that was identified in a previous screen as an inhibitor of angiogenesis. In this study, we conducted two animal model experiments to investigate the in vivo antiangiogenic activity of OMe-Syn. OMe-Syn significantly inhibited angiogenesis in a transgenic zebrafish model as well as in a mouse retinopathy model. To elucidate the underlying mechanisms responsible for the antiangiogenic activity of OMe-Syn, we used phage display cloning to isolate potential OMe-Syn binding proteins from human cDNA libraries and identified nucleoporin 153 kDa (NUP153) as a primary binding partner of OMe-Syn. OMe-Syn competitively inhibited mRNA binding to the RNA-binding domain of NUP153. Furthermore, depletion of NUP153 in human cells or zebrafish embryos led to an inhibition of angiogenesis, in a manner similar to that seen in response to OMe-Syn treatment. These data suggest that OMe-Syn is a promising candidate for the development of a novel antiangiogenic agent and that inhibition of NUP153 is possibly responsible for the antiangiogenic activity of OMe-Syn.

Therapeutic Potential of Anti-Angiogenic Multitarget N,O-Sulfated E. Coli K5 Polysaccharide in Diabetic Retinopathy

Vascular endothelial growth factor (VEGF) blockers have been developed for the treatment of proliferative diabetic retinopathy (PDR), the leading cause of visual impairments in the working-age population in the Western world. However, limitations to anti-VEGF therapies may exist because of the local production of other proangiogenic factors that may cause resistance to anti-VEGF interventions. Thus, novel therapeutic approaches targeting additional pathways are required. Here, we identified a sulfated derivative of the Escherichia coli polysaccharide K5 [K5-N,OS(H)] as a multitarget molecule highly effective in inhibiting VEGF-driven angiogenic responses in different in vitro, ex vivo, and in vivo assays, including a murine model of oxygen-induced retinopathy. Furthermore, K5-N,OS(H) binds a variety of heparin-binding angiogenic factors upregulated in PDR vitreous humor besides VEGF, thus inhibiting their biological activity. Finally, K5-N,OS(H) hampers the angiogenic activity exerted in vitro and in vivo by human vitreous fluid samples collected from patients with PDR. Together, the data provide compelling experimental evidence that K5-N,OS(H) represents an antiangiogenic multitarget molecule with potential implications for the therapy of pathologic neovessel formation in the retina of patients with PDR.

Gene Therapy with Endogenous Inhibitors of Angiogenesis for Neovascular Age-Related Macular Degeneration: Beyond Anti-VEGF Therapy

Age-related macular degeneration (AMD) is the leading cause of substantial and irreversible vision loss amongst elderly populations in industrialized countries. The advanced neovascular (or “wet”) form of the disease is responsible for severe and aggressive loss of central vision. Current treatments aim to seal off leaky blood vessels via laser therapy or to suppress vessel leakage and neovascular growth through intraocular injections of antibodies that target vascular endothelial growth factor (VEGF). However, the long-term success of anti-VEGF therapy can be hampered by limitations such as low or variable efficacy, high frequency of administration (usually monthly), potentially serious side effects, and, most importantly, loss of efficacy with prolonged treatment. Gene transfer of endogenous antiangiogenic proteins is an alternative approach that has the potential to provide long-term suppression of neovascularization and/or excessive vascular leakage in the eye. Preclinical studies of gene transfer in a large animal model have provided impressive preliminary results with a number of transgenes. In addition, a clinical trial in patients suffering from advanced neovascular AMD has provided proof-of-concept for successful gene transfer. In this mini review, we summarize current theories pertaining to the application of gene therapy for neovascular AMD and the potential benefits when used in conjunction with endogenous antiangiogenic proteins.

Targeting GRP78 and antiestrogen resistance in breast cancer

Breast cancer is the most prevalent cancer in women, with over 200,000 new cases diagnosed each year. Over 70% of breast cancers express the estrogen receptor-α, and drugs targeting these receptors such as tamoxifen or Faslodex(®) often fail to cure these patients. Many estrogen receptor-positive tumors lose drug sensitivity, making endocrine resistance a major clinical problem. Recently, investigation into the molecular mechanisms of endocrine resistance has highlighted a causative role of the unfolded protein response in antiestrogen resistance. In particular, the master regulator of the unfolded protein response, GRP78, was observed to be elevated in endocrine-resistant breast cancer and directly affected antiestrogen therapy responsiveness. GRP78 was found to impact many different cellular processes that may affect breast cancer survival. Recently, various compounds have been reported to affect GRP78 activity and it may be advantageous to combine these drugs with antiestrogens to overcome endocrine therapy resistance.

Antiangiogenic and antineuroinflammatory effects of kallistatin through interactions with the canonical Wnt pathway

Kallistatin is a member of the serine proteinase inhibitor superfamily. Kallistatin levels have been shown to be decreased in the vitreous while increased in the circulation of patients with diabetic retinopathy (DR). Overactivation of the Wnt pathway is known to play pathogenic roles in DR. To investigate the role of kallistatin in DR and in Wnt pathway activation, we generated kallistatin transgenic (kallistatin-TG) mice overexpressing kallistatin in multiple tissues including the retina. In the oxygen-induced retinopathy (OIR) model, kallistatin overexpression attenuated ischemia-induced retinal neovascularization. In diabetic kallistatin-TG mice, kallistatin overexpression ameliorated retinal vascular leakage, leukostasis, and overexpression of vascular endothelial growth factor and intracellular adhesion molecule. Furthermore, kallistatin overexpression also suppressed Wnt pathway activation in the retinas of the OIR and diabetic models. In diabetic Wnt reporter (BAT-gal) mice, kallistatin overexpression suppressed retinal Wnt reporter activity. In cultured retinal cells, kallistatin blocked Wnt pathway activation induced by high glucose and by Wnt ligand. Coprecipitation and ligand-binding assays both showed that kallistatin binds to a Wnt coreceptor LRP6 with high affinity (Kd = 4.5 nmol/L). These observations suggest that kallistatin is an endogenous antagonist of LRP6 and inhibitor of Wnt signaling. The blockade of Wnt signaling may represent a mechanism for its antiangiogenic and antineuroinflammatory effects.

Acidic/neutral amino acid residues substitution in NH2 terminal of plasminogen kringle 5 exerts enhanced effects on corneal neovascularization

PURPOSE

Recent results showed that plasminogen kringle 5 (K5) has improved inhibitory effect on human umbilical vein endothelial cells (HUVECs) viability when 5 acidic amino acids in NH2 terminal outside kringle domain were replaced by 5 serine residues (mutant K5, mK5). This study was designed to identify the enhanced antiangiogenic activity of mK5 in corneal neovascularization (CNV).

METHODS

Alkali burn-induced CNV was induced and treated with K5 and mK5 for 11 days. CNV and inflammation were evaluated by the CNV area and the inflammatory index, respectively. At the end of treatment, the corneas were removed for terminal deoxynucleotidyl transferase dUTP nick end labeling detection and immunohistochemistry. The effects of mK5 and K5 on HUVECs apoptosis were tested by MTT, BrdU, and flow cytometry. The expression levels of pigment epithelium-derived factor (PEDF) and vascular endothelial growth factor (VEGF) were detected by Western blot.

RESULTS

In a rat model of CNV induced by alkali, topical treatment with mK5 significantly decreased the neovascular area and inflammation compared with the wild-type K5-treated group. Meanwhile, mK5 and K5 specifically inhibited the HUVECs proliferation and induced vascular endothelial cell apoptosis in vitro and in vivo, and mK5 exerted higher apoptosis induction. Toward the mechanism of action, both mK5 and K5 significantly upregulated the expression of PEDF and mildly downregulated the expression of VEGF. The elevation of PEDF/VEGF ratio induced by mK5 was higher than that by K5.

CONCLUSIONS

These findings suggest that mK5 has more effective therapeutic potential in CNV than wild-type K5.

Dual inhibition of plasminogen kringle 5 on angiogenesis and chemotaxis suppresses tumor metastasis by targeting HIF-1α pathway

We had demonstrated that plasminogen kringle 5 (K5), a potent angiogenic inhibitor, inhibited retinal neovascularization and hepatocellular carcinoma growth by anti-angiogenesis. The current study investigated the effects and the underlying mechanisms of K5 on both tumor growth and spontaneous pulmonary metastasis in Lewis lung carcinoma (LLC) implanted mouse model. Similarly, K5 could decrease expression of VEGF in LLC cells and grafted tissues and suppress tumor angiogenesis and growth. K5 had no direct effect on proliferation and apoptosis of LLC. However, K5 could significantly inhibit SDF-1α-induced chemotaxis movement of LLC cells and resulted in a great reduction of surface metastatic nodules and micrometastases in the lungs of LLC tumor-bearing mice. K5 also decreased expression of chemokine (C-X-C motif) receptor 4 (CXCR4) in LLC cells and grafted tissues. Furthermore, K5 down-regulated SDF-1α expression in metastatic lung tissues of LLC-bearing mice. Therefore, K5 may suppress tumor pulmonary metastasis through inhibiting SDF-1α-CXCR4 chemotaxis movement and down-regulation of VEGF. Moreover, the role of hypoxia inducible factor-1α (HIF-1α), a crucial transcriptional factor for both VEGF and CXCR4 expression, was evaluated. The siRNA of HIF-1α attenuated expression of VEGF and CXCR4 and inhibited LLC migration. K5 decreased HIF-1α protein level and impaired nuclear HIF-1α accumulation. These results showed for the first time that K5 inhibits LLC growth and metastasis via the dual effects of anti-angiogenesis and suppression of tumor cell motility by targeting the pivotal molecule, HIF-1α.

Impacts of hypoxia-inducible factor-1 knockout in the retinal pigment epithelium on choroidal neovascularization

PURPOSE

Hypoxia-inducible factor (HIF)-1 is a key oxygen sensor and is believed to play an important role in neovascularization (NV). The purpose of this study is to determine the role of retinal pigment epithelium (RPE)-derived HIF-1α on ocular NV.

METHODS

Conditional HIF-1α knockout (KO) mice were generated by crossing transgenic mice expressing Cre in the RPE with HIF-1α floxed mice, confirmed by immunohistochemistry, Western blot analysis, and fundus fluorescein angiography. The mice were used for the oxygen-induced retinopathy (OIR) and laser-induced choroidal neovascularization (CNV) models.

RESULTS

HIF-1α levels were significantly decreased in the RPE layer of ocular sections and in primary RPE cells from the HIF-1α KO mice. Under normal conditions, the HIF-1α KO mice exhibited no apparent abnormalities in retinal histology or visual function as shown by light microscopy and electroretinogram recording, respectively. The HIF-1α KO mice with OIR showed no significant difference from the wild-type (WT) mice in retinal levels of HIF-1α and VEGF as well as in the number of preretinal neovascular cells. In the laser-induced CNV model, however, the disruption of HIF-1α in the RPE attenuated the over expression of VEGF and the intercellular adhesion molecule 1 (ICAM-1), and reduced vascular leakage and CNV area.

CONCLUSIONS

RPE-derived HIF-1α plays a key role in CNV, but not in ischemia-induced retinal NV.

Plasminogen K5 activates mitochondrial apoptosis pathway in endothelial cells by regulating Bak and Bcl-x(L) subcellular distribution

Plasminogen Kringle 5(K5) is a proteolytic fragment of plasminogen, which displays potent anti-angiogenic activities. K5 has been shown to induce apoptosis in proliferating endothelial cells; however the exact mechanism has not been well explored. The present study was designed to elucidate the possible molecular mechanism of K5-induced endothelial cell apoptosis. Our results showed that K5 inhibited basic fibroblast growth factors activated in human umbilical vein endothelial cells (HUVECs), indicating proliferation in a dose-dependent manner and induced endothelial cell death via apoptosis. K5 exposure activated caspase 7, 8 and 9. These results suggested that both the intrinsic mitochondrial apoptosis pathway and extrinsic pathway might be involved in K5-induced apoptosis. K5 reduced mitochondrial membrane potential (MMP) of HUVECs, demonstrating mitochondrial depolarization in HUVECs. K5 increased the ratio of Bak to Bcl-x(L) on mitochondria, decreased the ratio in cytosol, and had no effect on the total amounts of these proteins. K5 also did not effect on Bax/Bcl-2 distribution. K5 increased the ratio of Bak to Bcl-x(L) on mitochondrial that resulted in mitochondrial depolarization, cytochrome c release and consequently the cleavage of caspase 9. These results suggested that K5 induces endothelial cell apoptosis at least in part via activating mitochondrial apoptosis pathway. The regulation of K5 on Bak and Bcl-x(L) distribution may play an important role in endothelial cell apoptosis. These results provide further insight into the anti-angiogenesis roles of K5 in angiogenesis-related ocular diseases and solid tumors.

Integrin-mediated cell-matrix interaction in physiological and pathological blood vessel formation

Physiological as well as pathological blood vessel formation are fundamentally dependent on cell-matrix interaction. Integrins, a family of major cell adhesion receptors, play a pivotal role in development, maintenance, and remodeling of the vasculature. Cell migration, invasion, and remodeling of the extracellular matrix (ECM) are integrin-regulated processes, and the expression of certain integrins also correlates with tumor progression. Recent advances in the understanding of how integrins are involved in the regulation of blood vessel formation and remodeling during tumor progression are highlighted. The increasing knowledge of integrin function at the molecular level, together with the growing repertoire of integrin inhibitors which allow their selective pharmacological manipulation, makes integrins suited as potential diagnostic markers and therapeutic targets.

Anti-inflammatory and antiangiogenic effects of nanoparticle-mediated delivery of a natural angiogenic inhibitor

PURPOSE

The purpose of this study was to evaluate the inhibitory effects of the nanoparticle-mediated delivery of plasminogen kringle 5 (K5) on choroidal neovascularization (CNV) and retinal inflammation.

METHODS

CNV was induced by laser in adult rats. Nanoparticles with an expression plasmid of K5 (K5-NP) were injected into the vitreous. K5 expression was detected by immunohistochemistry. The CNV area was measured after fluorescein angiography. Retinal vascular permeability was quantified with Evans blue as a tracer. Expression of vascular endothelial growth factor (VEGF), tumor necrosis factor (TNF)-α, monocyte chemoattractant protein (MCP)-1, and intercellular adhesion molecule (ICAM)-1 was measured by Western blot analysis or ELISA and real-time RT-PCR.

RESULTS

Intense K5 expression was detected in the retina 2 weeks after the injection of K5-NP. Areas of CNV were significantly decreased in the K5-NP treatment group compared with that in the control-NP group. The K5-NP injection also significantly reduced vascular permeability. The expression of VEGF was downregulated by K5-NP at both the protein and mRNA levels. Moreover, K5-NP also inhibited expression of TNF-α and ICAM-1. Similarly, K5-NP decreased retinal levels of total β-catenin. In cultured cells, K5-NP suppressed hypoxia-induced secretion of MCP-1 and TNF-α.

CONCLUSIONS

K5 has a novel anti-inflammatory activity. K5-NP mediates a sustained inhibitory effect on CNV and thus has therapeutic potential for age-related macular degeneration.

Ischaemia-induced retinal neovascularisation and diabetic retinopathy in mice with conditional knockout of hypoxia-inducible factor-1 in retinal Müller cells

AIMS/HYPOTHESIS

Retinal Müller cells are known to produce inflammatory and angiogenic cytokines, which play important roles in diabetic retinopathy. Hypoxia-inducible factor (HIF)-1 has been shown to play a crucial role in retinal inflammation and neovascularisation. We sought to determine the role of Müller cell-derived HIF-1 in oxygen-induced retinopathy (OIR) and diabetic retinopathy using conditional Hif-1α (also known as Hif1a) knockout (KO) mice.

METHODS

Conditional Hif-1α KO mice were generated by crossing mice expressing cyclisation recombinase (cre, also known as P1_gp003) in Müller cells with floxed Hif-1α mice and used for OIR and streptozotocin-induced diabetes to induce retinal neovascularisation and inflammation, respectively. Abundance of HIF-1α and pro-angiogenic and pro-inflammatory factors was measured by immunoblotting and immunohistochemistry. Retinal neovascularisation was visualised by angiography and quantified by counting pre-retinal nuclei. Retinal inflammation was evaluated by leucostasis and vascular leakage.

RESULTS

While the Hif-1α KO mice showed significantly decreased HIF-1α levels in the retina, they exhibited no apparent histological or visual functional abnormalities under normal conditions. Compared with wild-type counterparts, Hif-1α KO mice with OIR demonstrated attenuated overproduction of vascular endothelial growth factor (VEGF) and intercellular adhesion molecule (ICAM)-1, reduced vascular leakage and alleviated neovascularisation in the retina. Under diabetes conditions, disruption of Hif-1α in Müller cells attenuated the increases of retinal vascular leakage and adherent leucocytes, as well as the overproduction of VEGF and ICAM-1.

CONCLUSIONS/INTERPRETATION

Müller cell-derived HIF-1α is a key mediator of retinal neovascularisation, vascular leakage and inflammation, the major pathological changes in diabetic retinopathy. Müller cell-derived HIF-1α is therefore a promising therapeutic target for diabetic retinopathy.

High efficacy and minimal peptide required for the anti-angiogenic and anti-hepatocarcinoma activities of plasminogen K5

Kringle 5(K5) is the fifth kringle domain of human plasminogen and its anti-angiogenic activity is more potent than angiostatin that includes the first four kringle fragment of plasminogen. Our recent study demonstrated that K5 suppressed hepatocarcinoma growth by anti-angiogenesis. To find high efficacy and minimal peptide sequence required for the anti-angiogenic and anti-tumour activities of K5, two deletion mutants of K5 were generated. The amino acid residues outside kringle domain of intact K5 (Pro452-Ala542) were deleted to form K5mut1(Cys462-Cys541). The residue Cys462 was deleted again to form K5mut2(Met463-Cys541). K5mut1 specifically inhibited proliferation, migration and induced apoptosis of endothelial cells, with an apparent two-fold enhanced activity than K5. Intraperitoneal injection of K5mut1 resulted in more potent tumour growth inhibition and microvessel density reduction than K5 both in HepA-grafted and Bel7402-xenografted hepatocarcinoma mouse models. These results suggested that K5mut1 has more potent anti-angiogenic activity than intact K5. K5mut2, which lacks only the amino terminal cysteine of K5mut1, completely lost the activity, suggesting that the kringle domain is essential for the activity of K5. The activity was enhanced to K5mut1 level when five acidic amino acids of K5 in NH₂ terminal outside kringle domain were replaced by five serine residues (K5mut3). The shielding effect of acidic amino acids may explain why K5mut1 has higher activity. K5, K5mut1 and K5mut3 held characteristic β-sheet spectrum while K5mut2 adopted random coil structure. These results suggest that K5mut1 with high efficacy is the minimal active peptide sequence of K5 and may have therapeutic potential in liver cancer.

The potential of nanomedicine therapies to treat neovascular disease in the retina

Neovascular disease in the retina is the leading cause of blindness in all age groups. Thus, there is a great need to develop effective therapeutic agents to inhibit and prevent neovascularization in the retina. Over the past decade, anti-VEGF therapeutic agents have entered the clinic for the treatment of neovascular retinal disease, and these agents have been effective for slowing and preventing the progression of neovascularization. However, the therapeutic benefits of anti-VEGF therapy can be diminished by the need for prolonged treatment regimens of repeated intravitreal injections, which can lead to complications such as endophthalmitis, retinal tears, and retinal detachment. Recent advances in nanoparticle-based drug delivery systems offer the opportunity to improve bioactivity and prolong bioavailability of drugs in the retina to reduce the risks associated with treating neovascular disease. This article reviews recent advances in the development of nanoparticle-based drug delivery systems which could be utilized to improve the treatment of neovascular disease in the retina.

Nanoparticle-mediated expression of an angiogenic inhibitor ameliorates ischemia-induced retinal neovascularization and diabetes-induced retinal vascular leakage

OBJECTIVE

The aim of the study is to evaluate the effect of nanoparticle-mediated gene delivery of angiogenic inhibitors on retinal inflammation, vascular leakage, and neovascularization in diabetic retinopathy.

RESEARCH DESIGN AND METHODS

An expression plasmid of plasminogen kringle 5 (K5), a natural angiogenic inhibitor, was encapsulated with poly(lactide-coglycolide) to form K5 nanoparticles (K5-NP). Expression of K5 was determined by Western blot analysis and immunohistochemistry, and retinal vascular leakage was measured by permeability assay. Retinal neovascularization was evaluated using fluorescein-angiography and counting preretinal vascular cells in rats with oxygen-induced retinopathy. Effects of K5-NP on retinal inflammation were evaluated in streptozotocin-induced diabetic rats by leukostasis assay and Western blot analysis of intracellular adhesion molecule and vascular endothelial growth factor. Possible toxicities of K5-NP were evaluated using histology examination, retinal thickness measurement, and electroretinogram recording.

RESULTS

K5-NP mediated efficient expression of K5 and specifically inhibited growth of endothelial cells. An intravitreal injection of K5-NP resulted in high-level expression of K5 in the inner retina of rats during the 4 weeks they were analyzed. Injection of K5-NP significantly reduced retinal vascular leakage and attenuated retinal neovascularization, when compared with the contralateral eyes injected with Control-NP in oxygen-induced retinopathy rats. K5-NP attenuated vascular endothelial growth factor and intracellular adhesion molecule-1 overexpression and reduced leukostasis and vascular leakage for at least 4 weeks after a single injection in the retina of streptozotocin-induced diabetic rats. No toxicities of K5-NP were detected to retinal structure and function.

CONCLUSIONS

K5-NP mediates efficient and sustained K5 expression in the retina and has therapeutic potential for diabetic retinopathy.

Update on Endogenous Inhibitors of Angiogenesis

Angiogenesis is regulated by a local balance between the levels of endogenous stimulators and inhibitors of angiogenesis. Understanding of the mechanism of angiogenesis has advanced significantly since the discovery of two members of the family of angiogenesis stimulators, i.e., vascular endothelial growth factor family proteins and angiopoietins. These factors act on endothelial cells to stimulate angiogenesis. In contrast, most of angiogenesis inhibitors do not seem to have such characteristics. Very few genes encoding molecules that selectively inhibit angiogenesis have been discovered. This review will focus on our current understanding of endogenous inhibitors of angiogenesis.

Systemic and Periocular Deliveries of Plasminogen Kringle 5 Reduce Vascular Leakage in Rat Models of Oxygen-Induced Retinopathy and Diabetes

PURPOSE

Increased retinal vascular permeability is a common complication of diabetes and a major cause of vision loss in diabetic patients. The current study is to determine the effect of plasminogen kringle 5 (K5) on vascular leakage via systemic and periocular deliveries.

METHODS

Oxygen-induced retinopathy (OIR) was generated by exposing newborn rats to 75% oxygen. Diabetes was induced in adult rats by injection of streptozotocin (STZ). Retinal vascular permeability was measured by the Evans blue-albumin leakage method.

RESULTS

Subcutaneous, intraperitoneal, subconjunctival, and retrobulbar injections and topical eyedrop application of K5 significantly reduced retinal vascular permeability in both the OIR and STZ-diabetic rat models. Compared with the periocular deliveries, systemic administration requires higher doses of K5. K5 deliveries downregulated VEGF expression in the retina.

CONCLUSIONS

K5 can reduce retinal vascular permeability through systemic and periocular deliveries. These delivery routes of K5 have therapeutic potential in the treatment of vascular leakage.

Plasminogen kringle 5 induces apoptosis of brain microvessel endothelial cells: sensitization by radiation and requirement for GRP78 and LRP1

Recombinant plasminogen kringle 5 (rK5) has been shown to induce apoptosis of dermal microvessel endothelial cells (MvEC) in a manner that requires glucose-regulated protein 78 (GRP78). As we are interested in antiangiogenic therapy for glioblastoma tumors, and the effectiveness of antiangiogenic therapy can be enhanced when combined with radiation, we investigated the proapoptotic effects of rK5 combined with radiation on brain MvEC. We found that rK5 treatment of brain MvEC induced apoptosis in a dose- and time-dependent manner and that prior irradiation significantly sensitized (500-fold) the cells to rK5-induced apoptosis. The rK5-induced apoptosis of both unirradiated and irradiated MvEC required expression of GRP78 and the low-density lipoprotein receptor-related protein 1 (LRP1), a scavenger receptor, based on down-regulation studies with small interfering RNA, and blocking studies with either a GRP78 antibody or a competitive inhibitor of ligand binding to LRP1. Furthermore, p38 mitogen-activated protein kinase was found to be a necessary downstream effector for rK5-induced apoptosis. These data suggest that irradiation sensitizes brain MvEC to the rK5-induced apoptosis and that this signal requires LRP1 internalization of GRP78 and the activation of p38 mitogen-activated protein kinase. Our findings suggest that prior irradiation would have a dose-sparing effect on rK5 antiangiogenic therapy for brain tumors and further suggest that the effects of rK5 would be tumor specific, as the expression of GRP78 protein is up-regulated on the brain MvEC in glioblastoma tumor biopsies compared with the normal brain.

Deguelin inhibits retinal neovascularization by down-regulation of HIF-1alpha in oxygen-induced retinopathy

Retinal neovascularization is the most common cause of blindness; Retinopathy of pre-maturity (ROP) for children and diabetic retinopathy for young age group. ROP still remains as the most serious cause of vision loss in children. We provided that deguelin significantly reduces retinal neovascularization in a mouse model of ROP. Deguelin never affected the transcriptional activity of hypoxia inducible factor (HIF)-1, however, reduced HIF-1 expression, which led to the decrease of vascular endothelial growth factor expression. Deguelin effectively suppressed endothelial cell proliferation without cytotoxic effect under therapeutic concentration range. In addition, deguelin demonstrated no reduction or retardation in normal retinal development and no retinal toxicity. These data suggest deguelin is a potent inhibitor of retinal neovascularization and may be applied in the treatment of other vasoproliferative retinopathies.

Suppression of retinal neovascularization by the iNOS inhibitor aminoguanidine in mice of oxygen-induced retinopathy

BACKGROUND

Retinal neovascularization (NV) is a major cause of blindness associated with ischemic retinal disorders. Our study was focused on evaluating the inhibitory effect of aminoguanidine (AG), an inhibitor of inducible nitric oxide synthase (iNOS), on retinal NV in mice of oxygen-induced retinopathy (OIR).

METHODS

An OIR model was established with 7-day-old C57BL/6J mice. One day before and 1 and 3 days after being returned to the room air, the right eyes were injected intravitreally with bevacizumab, AG or bevacizumab+AG respectively. The left eyes were injected with normal saline (NS) as control. The mice were killed at postnatal day 17 (P17). The effects of AG or bevacizumab on iNOS or VEGF expressions were evaluated by RT-PCR and immunohistochemistry. Retinal NV was examined by fluorescein angiography, and was quantified histologically by CD34 immnunostaining at P17.

RESULTS

Compared with NS-treated eyes, retinal VEGF and iNOS mRNA expressions were significantly reduced in AG- and bevacizumab+AG-treated eyes; whereas in bevacizumab-treated eyes, retinal VEGF mRNA expression increased and iNOS mRNA expression remained unchanged. The above changes were confirmed by immunohistochemical study. The generalized decrease in both VEGF and iNOS distributions in mice retina treated with AG or bevacizumab+AG was demonstrated by immunohistochemistry. Retinal NV was significantly reduced in all three groups treated with bevacizumab, AG or bevacizumab+AG, when compared with NS-treated eyes.

CONCLUSIONS

iNOS activation plays a pathological role in retinal NV in a mouse model of ischemic retinopathy. Administration of AG significantly suppressed retinal NV. Therefore, AG appears to be a novel and effective therapeutic approach for retinal NV.

New molecular targets in angiogenic vessels of glioblastoma tumours

Antiangiogenesis approaches have the potential to be particularly effective in the treatment of glioblastoma tumours. These tumours exhibit extremely high levels of neovascularisation, which may contribute to their extremely aggressive behaviour, not only by providing oxygenation and nutrition, but also by establishing a leaky vasculature that lacks a blood-brain barrier. This leaky vasculature enables migration of tumour cells, as well as the build up of fluid, which exacerbates tissue damage due to increased intracranial pressure. Here, we discuss the considerable progress that has been made in the identification of the pro- and antiangiogenic factors produced by glioblastoma tumours and the effects of these molecules in animal models of the disease. The safety and efficacy of some of these approaches have now been demonstrated in clinical trials. However, the ability of tumours to overcome these therapies and to re-establish angiogenesis requires further clinical research regarding potential multimodality therapies, as well as basic research into the regulation of angiogenesis by as yet unidentified factors. Optimisation of noninvasive procedures for monitoring of angiogenesis would greatly facilitate such research.

Increased suppression of oncolytic adenovirus carrying mutant k5 on colorectal tumor

Angiogenesis plays a key role in the development of a wide variety of malignant tumors. The approach of targeting antiangiogenesis has become an important field of cancer gene therapy. In this study, the antiangiogenesis protein K5 (the kringle 5 of human plasminogen) has been mutated by changing leucine71 to arginine to form mK5. Then the ZD55-mK5, which is an oncolytic adenovirus expressing mK5, was constructed. It showed stronger inhibition on proliferation of human umbilical vein endothelial cell. Moreover, in tube formation and embryonic chorioallantoic membrane assay, ZD55-mK5 exhibited more effective antiangiogenesis than ZD55-K5. In addition, ZD55-mK5 generated obvious suppression on the growth of colorectal tumor xenografts and prolonged the life span of nude mice. These results indicate that ZD55-mK5 is a potent agent for inhibiting the tumor angiogenesis and tumor growth.

Strategies for blocking angiogenesis in diabetic retinopathy: from basic science to clinical practice

Proliferative diabetic retinopathy (PDR) demands both more effective and less expensive biologically based treatments. Our understanding of the pathophysiology of the disease is increasing as new biochemical pathways are identified. Most reports emphasize proangiogenic stimuli, with the natural inhibitory elements receiving little attention. There are two therapeutic strategies for blocking retinal angiogenesis in PDR: systemic drug administration (protein kinase C inhibitors and somatostatin analogs) or local therapies (anti-vascular endothelial growth factor strategies, anti-inflammatory agents, gene therapy and stem cell therapy). This review mainly focuses on the role of local therapies, especially intravitreous delivery, in the management of PDR. The potential for adverse effect are also discussed. The availability of these new strategies or the combination of them will not only be beneficial in treating PDR but may also result in a shift towards treating earlier stages of diabetic retinopathy, thus easing the burden of this devastating disease.

Targeting matrix metalloproteinases and endothelial cells with a fusion peptide against tumor

Development of novel therapy for patients with tumor is still a challenge at the present time. We designed a fusion peptide (RK5) with two targets as a novel agent against tumor. The fusion peptide RK5 containing the kringle 5 fragment of human plasminogen and a decapeptide (CTTHWGFTLC) was constructed and expressed in yeast. Matrix metalloproteinase (MMP) activity, proliferation, and migration of endothelial cells were examined in vitro, respectively. Angiogenesis, tumor growth, metastasis, and survival time were evaluated in in vivo models. Administration of RK5 was delivered by both protein and gene approach. The results showed that RK5 inhibited the activity of MMP-9 and exhibited more inhibitory effects on proliferation and migration of endothelial cells than that of kringle 5 fragment and decapeptide individually. RK5 also inhibited angiogenesis, tumor growth, and metastasis and increased survival time of mice bearing tumor. In addition, the effectiveness of RK5 could be achieved by both protein and gene delivery. In conclusion, RK5 has potential to inhibit tumor growth and metastasis and to prolong survival time of animals bearing tumor. Therefore, fusion peptide RK5 with two targets provides a new design for the development of antitumor drugs and has potential for clinical application.

Homoisoflavanone inhibits retinal neovascularization through cell cycle arrest with decrease of cdc2 expression

Neovascularization in the eye is the most common cause of blindness in all age groups; retinopathy of prematurity (ROP), diabetic retinopathy, and age-related macular degeneration. Despite current advances in surgical treatments, ROP remains as the most serious problem of vision loss in children. Here, we report that homoisoflavanone, a natural product from Cremastra appendiculata, significantly reduces retinal neovascularization in a mouse model of ROP. Homoisoflavanone inhibited the cell growth of HUVECs, but its cytotoxic effect was not observed in a concentration range of 1-20 microM. HUVECs population gradually increased in G2/M phase and reduced in G0/G1 and S phases after exposure to the compound. Homoisoflavanone decreased the level of cdc2 expression whereas the level of p21WAF1 expression was increased in a dose-dependent manner. These data demonstrate that homoisoflavanone could inhibit retinal neovascularization and be applied in the treatment of other vasoproliferative retinopathies.

The role of clusterin in retinal development and free radical damage

AIM

To assess the role of clusterin in retinal vascular development and in free radical damage in vivo and in vitro.

METHODS

The expression of clusterin, von Willebrand factor (vWF), flk-1, heat shock protein 27 (Hsp27) and heat shock protein 70 (Hsp70) was examined in the retinas of developing mice and oxygen-induced retinopathy (OIR) mice by immunofluorescence staining and western blot analysis. Hydrogen peroxide (H(2)O(2))-pretreated human retinal endothelial cells (HREC) and astrocytes were cultured in the presence or absence of exogenous clusterin, and then the cell viability was measured using the MTT assay and DAPI staining.

RESULTS

Clusterin was expressed mainly in the inner retina and co-localised with vWF, an endothelial cell marker. During the mouse developmental process, clusterin expression was decreased, which was similar to the expression of flk-1, vWF and Hsp27. Furthermore, in the OIR model, clusterin expression changed in a similar way to both vWF and Hsp27. Under hypoxic conditions, clusterin expression increased in HREC and astrocytes. In H(2)O(2)-pretreated HREC and astrocytes, clusterin protected against apoptotic cell death.

CONCLUSIONS

These results suggest that clusterin is associated with protection from apoptotic retinal cell death in retinal development and in free radical damage.

Bone Marrow Stromal Cells Act as Feeder Cells for Tendon Fibroblasts through Soluble Factors

Feeder effects of bone marrow stromal cells (BMSCs) on tendon fibroblasts were investigated using a co-culture method for the application of ligament or tendon tissue engineering and cell therapy. BMSCs had significant effects on enhancing cell proliferation, the ability of cells to migrate, and cell adhesivity but little effect on the extracellular matrix (ECM) synthesis of tendon fibroblasts without cell-cell contact. Furthermore, the conditioned medium from BMSCs, despite the existence of fibroblasts, significantly increased the number of fibroblasts. Based on these results, the mechanism of the feeder effects is considered to be a certain signal of soluble factors from BMSCs to the fibroblasts. Comparative proteome analysis of the conditioned medium from co-culture of fibroblasts and BMSCs revealed less expression of plasminogen, which showed inhibitory effects on fibroblast proliferation. With regard to the relationships between plasminogen and BMSCs in the co-culture system, we speculate that BMSCs allow resolution of plasminogen or its cleavage activity in the medium via some mechanism.

Ocular neovascularization: Implication of endogenous angiogenic inhibitors and potential therapy

Ocular neovascularization (NV) is the primary cause of blindness in a wide range of ocular diseases, such as diabetic retinopathy (DR) and age-related macular degeneration (AMD). The exact mechanism underlying the pathogenesis of ocular NV is not yet well understood, and as a consequence, there is no satisfactory therapy for ocular NV. In the last 10 years, a number of studies provided increasing evidence demonstrating that the imbalance between angiogenic stimulating factors and angiogenic inhibitors is a major contributor to the angiogenesis induced by various insults, such as hypoxia or ischemia, inflammation and tumor. The angiogenic inhibitors alone or in combination with other existing therapies are, therefore, believed to be promising in the treatment of ocular NV in the near future. This article reviews recent progress in studies on the mechanisms and treatment of ocular NV, focusing on the implication and therapeutic potential of endogenous angiogenic inhibitors in ocular NV.

Enhanced anti-angiogenic effect of a deletion mutant of plasminogen kringle 5 on neovascularization

Kringle 5 (K5), a proteolytic fragment of plasminogen, has been proved to be an angiogenic inhibitor. Previously, we have evaluated the effect of K5 on the vascular leakage and neovascularization in a rat model of oxygen-induced retinopathy. In this study, we expressed K5 and a deletion mutant of K5 (K5 mutant) in a prokaryocyte expression system and purified them by affinity chromatography. K5 mutant was generated by deleting 11 amino acids from K5 while retaining the three disulfide bonds. The anti-angiogenic activity of intact K5 and K5 mutant were compared in endothelial cells and retinal neovascularization rat model. K5 mutant inhibited the proliferation of primary human retinal capillary endothelial cells (HRCEC) in a concentration-dependent manner, with an apparent EC50 of approximate 35 nmol/L, which is twofold more potent than intact K5. In the even higher concentration range, K5 mutant did not inhibit pericytes from the same origin of HRCEC, which suggested an endothelial cell-specific inhibition. K5 mutant had no effect on normal liver cells and Bel7402 hepatoma cells even at high concentration range either. Intravitreal injection of the K5 and mutant in the oxygen-induced retinopathy rat model both resulted in significantly fewer neovascular tufts and nonperfusion area than controls with PBS injection, as shown by fluorescein angiography. Furthermore, K5 mutant exhibited more strong inhibition effect on neovascularization than intact K5 by quantification of vascular cells. These results suggest that this K5 deletion mutant is a more potent angiogenic inhibitor than intact K5 and may have therapeutic potential in the treatment of those disorders with neovascularization, such as solid tumor, diabetic retinopathy, age-related macular degeneration, rheumatoid arthritis, and hyperplasia of prostate.

Angiostatin's molecular mechanism: aspects of specificity and regulation elucidated

Tumor growth requires the development of new vessels that sprout from pre-existing normal vessels in a process known as "angiogenesis" [Folkman (1971) N Engl J Med 285:1182-1186]. These new vessels arise from local capillaries, arteries, and veins in response to the release of soluble growth factors from the tumor mass, enabling these tumors to grow beyond the diffusion-limited size of approximately 2 mm diameter. Angiostatin, a naturally occurring inhibitor of angiogenesis, was discovered based on its ability to block tumor growth in vivo by inhibiting the formation of new tumor blood vessels [O'Reilly et al. (1994a) Cold Spring Harb Symp Quant Biol 59:471-482]. Angiostatin is a proteolytically derived internal fragment of plasminogen and may contain various members of the five plasminogen "kringle" domains, depending on the exact sites of proteolysis. Different forms of angiostatin have measurably different activities, suggesting that much remains to be elucidated about angiostatin biology. A number of groups have sought to identify the native cell surface binding site(s) for angiostatin, resulting in at least five different binding sites proposed for angiostatin on the surface of endothelial cells (EC). This review will consider the data supporting all of the various reported angiostatin binding sites and will focus particular attention on the angiostatin binding protein identified by our group: F(1)F(O) ATP synthase. There have been several developments in the quest to elucidate the mechanism of action of angiostatin and the regulation of its receptor. The purpose of this review is to describe the highlights of research on the mechanism of action of angiostatin, its' interaction with ATP synthase on the EC surface, modulators of its activity, and issues that should be explored in future research related to angiostatin and other anti-angiogenic agents.

Kringle 5 of human plasminogen induces apoptosis of endothelial and tumor cells through surface-expressed glucose-regulated protein 78

Kringle 5 (K5) of human plasminogen has been shown to inhibit angiogenesis by inducing the apoptosis of proliferating endothelial cells. Peptide regions around the lysine-binding pocket of K5 largely mediate these effects, particularly the peptide PRKLYDY, which we show to compete with K5 for the binding to endothelial cells. The cell surface binding site for K5 that mediates these effects has not been defined previously. Here, we report that glucose-regulated protein 78, exposed on cell surfaces of proliferating endothelial cells as well as on stressed tumor cells, plays a key role in the antiangiogenic and antitumor activity of K5. We also report that recombinant K5-induced apoptosis of stressed HT1080 fibrosarcoma cells involves enhanced activity of caspase-7, consistent with the disruption of glucose-regulated protein 78-procaspase-7 complexes. These results establish recombinant K5 as an inhibitor of a stress response pathway, which leads to both endothelial and tumor cell apoptosis.

Endogenous inhibitors of angiogenesis

Angiogenesis, the formation of new blood vessels, is required for many pathologic processes, including invasive tumor growth as well as physiologic organ/tissue maintenance. Angiogenesis during development and adulthood is likely regulated by a balance between endogenous proangiogenic and antiangiogenic factors. It is speculated that tumor growth requires disruption of such balance; thus, the angiogenic switch must be turned "on" for cancer progression. If the angiogenic switch needs to be turned on to facilitate the tumor growth, the question remains as to what the physiologic status of this switch is in the adult human body; is it "off," with inhibitors outweighing the stimulators, or maintained at a fine "balance," keeping the proangiogenic properties of many factors at a delicate "activity" balance with endogenous inhibitors of angiogenesis. The physiologic status of this balance is important to understand as it might determine an individual's predisposition to turn the switch on during pathologic events dependent on angiogenesis. Conceivably, if the physiologic angiogenesis balance in human population exists somewhere between off and even balance, an individual's capacity and rate to turn the switch on might reflect their normal physiologic angiogenic status. In this regard, although extensive knowledge has been gained in our understanding of endogenous growth factors that stimulate angiogenesis, the activities associated with endogenous inhibitors are poorly understood. In this review, we will present an overview of the knowledge gained in studies related to the identification and characterization of 27 different endogenous inhibitors of angiogenesis.

Expression of biologically active kringle 5 domain of human plasminogen in Escherichia coli

The kringle 5 domain of plasminogen exhibits potent inhibitory effect on endothelial cell proliferation. It can also cause cell cycle arrest and apoptosis of endothelial cell specifically, and shows promise in anti-angiogenic therapy. It has been prepared via both proteolysis of native plasminogen and recombinant DNA methodologies. When previously expressed in Escherichia coli, recombinant kringle 5 mainly deposited as inactive, insoluble inclusion bodies and the refolding yield was low. In the present study, human kringle 5 was fusion-expressed with GST (gluthathione-S-transferase) under the control of T7 promoter in E. coli. The IPTG-induced GST-kringle 5 was about 20% of the total cellular proteins and, among the expressed GST-kringle 5 proteins, 80% was present in the supernatant. The GST-kringle 5 fusion protein exhibited some anti-proliferation activity towards bovine capillary endothelial cells. After GST-kringle 5 purification, subsequent enterokinase release of intact kringle 5 from the fusion protein and further purification by gluthathione-Sepharose 4B affinity chromatography, the recombinant kringle 5, with a yield of 10.5 mg/L culture, displayed apparent inhibition of endothelial cell proliferation in a dose-dependent manner with ED50 about 20 nM.

High level expression of kringle 5 fragment of plasminogen in Pichia pastoris

Angiogensis can be blocked by inhibitors such as endostatin and angiostatin. The kringle 5 fragment of plasminogen also has a potent inhibitory effect on endothelial cell proliferation and leads to the inhibition of angiogenesis. It has promise in anti-angiogenic therapy due to its small size and potent inhibitory effect. Preparation of kringle 5 has been achieved through the proteolysis of native plasminogen and recombinant DNA technology. Bacterially expressed recombinant kringle 5 is mainly insoluble and expressed at low level. The refolding yield is also low. To produce recombinant human kringle 5 in a large quantity, we have genetically modified a strain of Pichia pastoris. On methanol induction, this strain expressed and secreted biologically active, recombinant kringle 5. The expression level of the engineered strain in culture reached more than 300 mgl(-1). Purification was easily achieved by precipitation, hydrophobic and DEAE ion exchange chromatography. The recovery of recombinant kringle 5 was about 50% after purification. Yeast-expressed kringle 5 has a higher activity in anti-endothelial proliferation than bacterially expressed kringle 5.

Genetic difference in susceptibility to the blood-retina barrier breakdown in diabetes and oxygen-induced retinopathy

The breakdown of the blood-retina barrier (BRB) is a common feature of diabetic retinopathy. The purpose of the present study is to determine whether there are genetic differences in susceptibility to the breakdown of the BRB in diabetic retinopathy using two rat models. In streptozotocin (STZ)-induced diabetes, Brown Norway (BN) rats developed sustained vascular hyperpermeability in the retina during the entire experimental period (16 weeks of diabetes), while diabetic Sprague Dawley (SD) rats only showed retinal hyperpermeability from 3 to 10 days after the onset of diabetes. The strain difference in permeability was not correlated with the blood glucose levels in these two strains. In oxygen-induced retinopathy (OIR), BN rats developed retinal vascular hyperpermeability from postnatal day 12 (P12) to P22 with a peak at P16, which was 8.7-fold higher than that in the age-matched normal controls. In OIR-SD rats, however, hyperpermeability was observed from P14 to P18, with a peak only 2.2-fold higher than that in the controls. The strain difference in vascular hyperpermeability was correlated with the different overexpression of vascular endothelial growth factor (VEGF) in the retina of these two models. This finding suggests that genetic backgrounds contribute to the susceptibility to diabetic retinopathy.

The effect of angiostatin on vascular leakage and VEGF expression in rat retina

Angiostatin is a potent angiogenic inhibitor. The present study identified a new activity of angiostatin: reducing vascular leakage, which is associated with diabetic macular edema, tumor growth and inflammation. An intravitreal injection of angiostatin significantly reduced retinal vascular permeability in rats with oxygen-induced retinopathy and in those with streptozotocin-induced diabetes, but not in normal rats. Consistent with its effect on permeability, angiostatin downregulated vascular endothelial growth factor (VEGF) expression in the retina in both the rat models but not in normal controls. These results suggest that the effect of angiostatin on vascular leakage is mediated, at least in part, through blockade of VEGF overexpression.

Gene transfer of kringle 5 of plasminogen by electroporation inhibits corneal neovascularization

PURPOSE

To test the efficacy of naked plasmid that expresses human kringle 5 of plasminogen (K5) in suppressing experimental corneal neovascularization in a rat model.

METHODS

A eukaryotic expression plasmid encoding human K5 (pSecK5) was constructed. COS cells were transiently transfected with pSecK5 using a lipid-based transfection reagent. K5 secretion was confirmed by Western blot analysis. The effect of the secreted K5 on the proliferation of human umbilical vein endothelial cells (HUVECs) was investigated colorimetrically. Forty-three Sprague-Dawley rats were used for a corneal neovascularization suppression experiment. Corneal injury was induced by placing a disk of filter paper (immersed in 1 mol/l NaOH, 3.0 mm in diameter) on the corneal surface for 2 min. The cornea was immediately washed with saline. pSecK5 and empty plasmids were injected subconjunctivally, and square-wave electric pulses were immediately applied to the eyes. The expression of K5 was analyzed by reverse transcription polymerase chain reaction (RT-PCR) and immunohistochemistry. The extent of corneal neovascularization was evaluated by scores.

RESULTS

The constructed plasmid could express itself in COS cells. Conditioned medium from K5-transfected COS cells apparently inhibited HUVEC proliferation, compared with conditioned medium from COS cells transfected with empty plasmid or nontransfected cells. RT-PCR and immunohistochemistry confirmed the expression of K5 in the conjunctiva and cornea. Corneal neovascularization was significantly suppressed by K5 gene transfer in rats' eyes.

CONCLUSION

In a rat model, K5 gene transfer by subconjunctival injection and electroporation can effectively inhibit corneal neovascularization induced by an alkali burn.

Decreased pigment epithelium-derived factor and increased vascular endothelial growth factor levels in pterygia

PURPOSE

Pterygia are histologically composed of proliferating fibrovascular tissue. This study compared expression levels of an angiogenic inhibitor, pigment epithelium-derived factor (PEDF), in pterygia with those in normal corneal and conjunctival tissues.

METHODS

The normal human conjunctival and corneal tissues were obtained from surgery or from donor eyes without ocular diseases. Pterygia were excised by therapeutic surgery under a microscope. Pigment epithelium-derived factor and vascular endothelial growth factor (VEGF) were measured by Western blot analysis. Their cellular localizations were determined by immunohistochemistry.

RESULTS

Intensive PEDF immunostaining was detected in all the normal corneal and conjunctival samples analyzed, predominantly in the epithelium and endothelium of the cornea and in the epithelium of the limbus and conjunctiva. Under the same immunostaining conditions, pterygial samples showed negative or faint PEDF staining. In contrast, the same pterygial samples all showed intensive VEGF staining, predominantly in the epithelium and in blood vessels. Western blot analysis confirmed that the average PEDF level in pterygia was drastically lower than those in normal corneal and conjunctival tissues, respectively. In contrast, the VEGF level in pterygia was significantly higher than in the normal tissues.

CONCLUSION

Pterygia exhibit significantly lower PEDF but higher VEGF levels than those in normal corneas and conjunctivae. The decreased PEDF level in pterygia may play a role in the formation and progression of pterygia.

Structural elements of kallistatin required for inhibition of angiogenesis

Kallistatin is a serpin first identified as a specific inhibitor of tissue kallikrein. Our recent studies showed that kallikrein promoted angiogenesis, whereas kallistatin inhibited angiogenesis and tumor growth. This study is aimed to identify the structural elements of kallistatin essential for its antiangiogenic function. Kallistatin mutants at the hinge region (A377T) and a major heparin-binding domain (K312A/K313A) were created by site-directed mutagenesis. Recombinant kallistatin mutant A377T did not bind or inhibit tissue kallikrein activity. Wild-type kallistatin and kallistatin mutant A377T, but not kallistatin mutant K312A/K313A lacking heparin-binding activity, inhibited VEGF-induced proliferation, growth, and migration of human microvascular endothelial cells. Similarly, wild-type kallistatin and kallistatin mutant A337T, but not kallistatin mutant K312A/K313A, significantly inhibited VEGF-induced capillary tube formation of cultured endothelial cells in Matrigel and capillary formation in Matrigel implants in mice. To elucidate the role of the heparin-binding domain in modulating angiogenesis, we showed that wild-type kallistatin interrupted the binding of (125)I-labeled VEGF to endothelial cells, whereas kallistatin mutant K312A/K313A did not interfere with VEGF binding. Consequently, wild-type kallistatin, but not kallistatin mutant K312A/K313A, suppressed VEGF-induced phosphorylation of Akt. Taken together, these results indicate that the heparin-binding domain, but not the reactive site loop of kallistatin, is essential for inhibiting VEGF-induced angiogenesis.

Kallikrein-binding protein inhibits retinal neovascularization and decreases vascular leakage

AIMS/HYPOTHESIS

Kallikrein-binding protein (KBP) is a serine proteinase inhibitor (serpin). It specifically binds to tissue kallikrein and inhibits kallikrein activity. Our study was designed to test its effects on retinal neovascularization and vascular permeability.

METHODS

Endothelial cell proliferation was determined by [(3)H] thymidine incorporation assay and apoptosis quantified by Annexin V staining and flow cytometry. Effect on retinal neovascularization was determined by fluorescein angiography and count of pre-retinal vascular cells in an oxygen-induced retinopathy (OIR) model. Vascular permeability was assayed by the Evans blue method. Vascular endothelial growth factor (VEGF) was measured by Western blot analysis and ELISA.

RESULTS

Kallikrein-binding protein specifically inhibited proliferation and induced apoptosis in retinal capillary endothelial cells. Intravitreal injection of KBP inhibited retinal neovascularization in an OIR model. Moreover, KBP decreased vascular leakage in the retina, iris and choroid in rats with OIR. Blockade of kinin receptors by specific antagonists showed significantly weaker inhibition of endothelial cells, when compared to that of KBP, suggesting that the anti-angiogenic activity of KBP is not through inhibiting kallikrein activity or kinin production. KBP competed with (125)I-VEGF for binding to endothelial cells and down-regulated VEGF production in endothelial cells and in the retina of the OIR rat model.

CONCLUSION/INTERPRETATION

Kallikrein-binding protein is a multi-functional serpin, and its vascular activities are independent of its interactions with the kallikrein-kinin system. Inhibition of VEGF binding to its receptors and down-regulation of VEGF expression could represent a mechanism for the vascular activities of KBP.