Expression and cellular localization of the kallikrein-kinin system in human ocular tissues

The vasoactive effects of bradykinin, vasoactive intestinal peptide, calcitonin gene-related peptide and neuropeptide Y depend on the perivascular tissue in porcine retinal arterioles in vitro

PURPOSE

The retina contains a number of vasoactive neuropeptides and corresponding receptors, but the role of these neuropeptides for tone regulation of retinal arterioles has not been studied in detail.

METHODS

Porcine arterioles with preserved perivascular retinal tissue were mounted in a wire myograph, and the tone was measured after the addition of increasing concentrations of bradykinin, vasoactive intestinal peptide (VIP), neuropeptide Y (NPY), substance P (SP), calcitonin gene-related peptide (CGRP) and brain natriuretic peptide (BNP). The experiments were performed during inhibition of the synthesis of nitric oxide (NO), prostaglandins and dopamine and were repeated after removal of the perivascular retinal tissue.

RESULTS

Bradykinin, VIP and CGRP induced significant concentration-dependent dilatation and NPY significant concentration-dependent contraction of the arterioles in the presence of perivascular retinal tissue (p < 0.03 for all comparisons) but not on isolated arterioles. BNP and SP had no effect on vascular tone. The NOS inhibitor L-NAME reduced bradykinin- and VIP-induced relaxation (p < 0.001 for both comparisons), whereas none of the other inhibitors influenced the vasoactive effects of the studied neuropeptides.

CONCLUSION

The effects of neuropeptides on the tone of retinal arterioles depend on the perivascular retinal tissue and may involve effects other than those mediated by nitric oxide, prostaglandins and adrenergic compounds. Investigation of the mechanisms underlying the vasoactive effect of neuropeptides may be important for understanding and treating retinal diseases where disturbances in retinal flow regulation are involved in the disease pathogenesis.

Current and Novel Therapeutic Approaches for Treatment of Diabetic Macular Edema

Diabetic macular edema (DME) is a major ocular complication of diabetes mellitus (DM), leading to significant visual impairment. DME's pathogenesis is multifactorial. Focal edema tends to occur when primary metabolic abnormalities lead to a persistent hyperglycemic state, causing the development of microaneurysms, often with extravascular lipoprotein in a circinate pattern around the focal leakage. On the other hand, diffusion edema is due to a generalized breakdown of the inner blood-retinal barrier, leading to profuse early leakage from the entire capillary bed of the posterior pole with the subsequent extravasation of fluid into the extracellular space. The pathogenesis of DME occurs through the interaction of multiple molecular mediators, including the overexpression of several growth factors, including vascular endothelial growth factor (VEGF), insulin-like growth factor-1, angiopoietin-1, and -2, stromal-derived factor-1, fibroblast growth factor-2, and tumor necrosis factor. Synergistically, these growth factors mediate angiogenesis, protease production, endothelial cell proliferation, and migration. Treatment for DME generally involves primary management of DM, laser photocoagulation, and pharmacotherapeutics targeting mediators, namely, the anti-VEGF pathway. The emergence of anti-VEGF therapies has resulted in significant clinical improvements compared to laser therapy alone. However, multiple factors influencing the visual outcome after anti-VEGF treatment and the presence of anti-VEGF non-responders have necessitated the development of new pharmacotherapies. In this review, we explore the pathophysiology of DME and current management strategies. In addition, we provide a comprehensive analysis of emerging therapeutic approaches to the treatment of DME.

Therapeutic Drugs and Devices for Tackling Ocular Hypertension and Glaucoma, and Need for Neuroprotection and Cytoprotective Therapies

Damage to the optic nerve and the death of associated retinal ganglion cells (RGCs) by elevated intraocular pressure (IOP), also known as glaucoma, is responsible for visual impairment and blindness in millions of people worldwide. The ocular hypertension (OHT) and the deleterious mechanical forces it exerts at the back of the eye, at the level of the optic nerve head/optic disc and lamina cribosa, is the only modifiable risk factor associated with glaucoma that can be treated. The elevated IOP occurs due to the inability of accumulated aqueous humor (AQH) to egress from the anterior chamber of the eye due to occlusion of the major outflow pathway, the trabecular meshwork (TM) and Schlemm’s canal (SC). Several different classes of pharmaceutical agents, surgical techniques and implantable devices have been developed to lower and control IOP. First-line drugs to promote AQH outflow via the uveoscleral outflow pathway include FP-receptor prostaglandin (PG) agonists (e.g., latanoprost, travoprost and tafluprost) and a novel non-PG EP2-receptor agonist (omidenepag isopropyl, Eybelis^®). TM/SC outflow enhancing drugs are also effective ocular hypotensive agents (e.g., rho kinase inhibitors like ripasudil and netarsudil; and latanoprostene bunod, a conjugate of a nitric oxide donor and latanoprost). One of the most effective anterior chamber AQH microshunt devices is the Preserflo^® microshunt which can lower IOP down to 10–13 mmHg. Other IOP-lowering drugs and devices on the horizon will be also discussed. Additionally, since elevated IOP is only one of many risk factors for development of glaucomatous optic neuropathy, a treatise of the role of inflammatory neurodegeneration of the optic nerve and retinal ganglion cells and appropriate neuroprotective strategies to mitigate this disease will also be reviewed and discussed.

Kinins and Their Receptors as Potential Therapeutic Targets in Retinal Pathologies

The kallikrein-kinin system (KKS) contributes to retinal inflammation and neovascularization, notably in diabetic retinopathy (DR) and neovascular age-related macular degeneration (AMD). Bradykinin type 1 (B1R) and type 2 (B2R) receptors are G-protein-coupled receptors that sense and mediate the effects of kinins. While B2R is constitutively expressed and regulates a plethora of physiological processes, B1R is almost undetectable under physiological conditions and contributes to pathological inflammation. Several KKS components (kininogens, tissue and plasma kallikreins, and kinin receptors) are overexpressed in human and animal models of retinal diseases, and their inhibition, particularly B1R, reduces inflammation and pathological neovascularization. In this review, we provide an overview of the KKS with emphasis on kinin receptors in the healthy retina and their detrimental roles in DR and AMD. We highlight the crosstalk between the KKS and the renin-angiotensin system (RAS), which is known to be detrimental in ocular pathologies. Targeting the KKS, particularly the B1R, is a promising therapy in retinal diseases, and B1R may represent an effector of the detrimental effects of RAS (Ang II-AT1R).

Role of Arginase in Selective Impairment of Endothelium-Dependent Nitric Oxide Synthase-Mediated Dilation of Retinal Arterioles during Early Diabetes

Purpose

Retinal vasomotor activity can be regulated by two major endothelial enzymes, nitric oxide synthase (NOS) and cyclooxygenase (COX). The vascular arginase also consumes a NOS substrate and thus impedes NOS-mediated vasodilation. Diabetes mellitus exhibits vascular complications in the retina with elevated oxidative stress and compromised NOS-mediated vasodilation. However, the underlying molecular mechanisms remain unclear, and the effect of diabetes on COX-mediated vasodilation is unknown. Herein, we examined the relative impact of diabetes on retinal arteriolar dilations to COX and NOS activation and the roles of arginase and superoxide in diabetes-induced vasomotor dysfunction.

Methods

Retinal arterioles were isolated from streptozocin-induced diabetic pigs (2 weeks of hyperglycemia, 433 ± 27 mg/dL) or age-matched control pigs (97 ± 4 mg/dL). The vasodilations to bradykinin (NOS activator) and histamine (NOS/COX activator) were examined in vitro.

Results

Retinal arteriolar dilations to histamine and bradykinin were significantly reduced after 2 weeks of diabetes. The NOS inhibitor N^G-nitro-L-arginine methyl ester (L-NAME) attenuated the dilations of control vessels, but not diabetic vessels, to histamine. In the presence of L-NAME and COX inhibitor indomethacin, histamine-induced dilations of control and diabetic vessels were reduced similarly. Treatment of diabetic vessels with arginase inhibitor nor-NOHA, but not superoxide dismutase mimetic TEMPOL, preserved both histamine- and bradykinin-induced dilations in an L-NAME-sensitive manner.

Conclusions

Arginase, rather than superoxide, impairs endothelium-dependent NOS-mediated dilation of retinal arterioles during diabetes, whereas vasodilation mediated by COX remains intact. Blockade of vascular arginase may improve endothelial function of retinal arterioles during early onset of diabetes.

Ophthalmic artery resistance index after peribulbar block in the presence of epinephrine

PURPOSE

There are controversies regarding ophthalmic artery (OA) flow after peribulbar block in the presence of epinephrine. Therefore, we aimed to evaluate OA flow via echo-Doppler before and after peribulbar block with lidocaine in the presence or absence of epinephrine.

METHODS

Fifty-six patients who had an American Society of Anesthesiologists (ASA) classification of I, II or III and were eligible for cataract phacoemulsification surgery were selected. Patients with other eye diseases were excluded. Patients were divided into two groups: group 1-peribulbar block with lidocaine and 1/200,000 epinephrine; group 2-peribulbar block with lidocaine in the absence of epinephrine. The resistance index (RI), peak systolic velocity (PSV), end diastolic velocity (EDV) of the OA were evaluated using echo-Doppler before and 10 min after the peribulbar block.

RESULTS

No differences between groups were observed in the RI before the peribulbar block as well regarding the presence of hypertension and the age or gender of the patient. After the peribulbar block, we observed a decrease in the RI in group 1 (p = 0.038, Cohen's d = 0.336) and no difference in the RI in group 2 (p = 0.109, Cohen's d = 0.172). When comparing group 1 and group 2, we observed a decrease in the RI in group 1 (p = 0.028, Cohen's d = 0,583). There was no difference between groups regarding the PSV and EDV after the peribulbar block.

CONCLUSIONS

A decrease in RI was observed in the OA after peribulbar block with a vasoconstrictor, an effect that could be a benefit in some ocular surgeries.

The effects of anti-VEGF and kinin B1 receptor blockade on retinal inflammation in laser-induced choroidal neovascularization

BACKGROUND AND PURPOSE

Age-related macular degeneration (AMD) is a complex neurodegenerative disease treated by anti-VEGF intravitreal injections. As inflammation is potentially involved in retinal degeneration, the pro-inflammatory kallikrein-kinin system is a possible alternative pharmacological target. Here, we investigated the effects of anti-VEGF and anti-B₁ receptor treatments on the inflammatory mechanisms in a rat model of choroidal neovascularization (CNV).

EXPERIMENTAL APPROACH

Immediately after laser-induced CNV, Long-Evans rats were treated by eye-drop application of a B₁ receptor antagonist (R-954) or by intravitreal injection of B₁ receptor siRNA or anti-VEGF antibodies. Effects of treatments on gene expression of inflammatory mediators, CNV lesion regression and integrity of the blood-retinal barrier was measured 10 days later in the retina. B₁ receptor and VEGF-R2 cellular localization was assessed.

KEY RESULTS

The three treatments significantly inhibited the CNV-induced retinal changes. Anti-VEGF and R-954 decreased CNV-induced up-regulation of B₁ and B₂ receptors, TNF-α, and ICAM-1. Anti-VEGF additionally reversed up-regulation of VEGF-A, VEGF-R2, HIF-1α, CCL2 and VCAM-1, whereas R-954 inhibited gene expression of IL-1β and COX-2. Enhanced retinal vascular permeability was abolished by anti-VEGF and reduced by R-954 and B₁ receptor siRNA treatments. Leukocyte adhesion was impaired by anti-VEGF and B₁ receptor inhibition. B₁ receptors were found on astrocytes and endothelial cells.

CONCLUSION AND IMPLICATIONS

B₁ receptor and VEGF pathways were both involved in retinal inflammation and damage in laser-induced CNV. The non-invasive, self-administration of B₁ receptor antagonists on the surface of the cornea by eye drops might be an important asset for the treatment of AMD.

Retinal proteome associated with bradykinin-induced edema

The plasma kallikrein-stimulated generation of bradykinin (BK) has been implicated in diabetic macular edema (DME). This study characterizes the effects of BK on the ultrastructure and proteome of the rat retina. The effects of intravitreal injection of BK on retinal thickness and vascular ultrastructure in Sprague Dawley rats were analyzed and compared with the effects of VEGF using spectral-domain optical coherence tomography. At 24 h post intravitreal injection of BK or saline vehicle retina were harvested and solubilized proteins were analyzed by mass spectrometry-based proteomics. Proteins were identified using X!Tandem and spectral counts were used as a semiquantitative measurement of protein abundance. Proteins identified from retinal extracts were annotated by Gene Ontology (GO) slim terms and compared with a human DME vitreous proteome. Intravitreal injection of BK and VEGF induced transient increases in retinal thickness of 46 μm (24.6%, p = 0.015) and 39 μm (20.3%, p = 0.004), respectively at 24 h, which were resolved to baseline thicknesses at 96 h post injection. BK and VEGF also increased retinal vessel diameters and tortuosity at 24 h post intravitreal injection. Proteomic analyses identified 1757 non-redundant proteins in the rat retina, including 1739 and 1725 proteins from BK- and saline control-injected eyes, respectively. Eighteen proteins, including two proteins associated with intercellular junctions, filamin A and actinin alpha 4, were decreased by at least 50% (p < 0.05) in retina from BK-injected eyes compare with retina from eyes injected with saline. In addition, 32 proteins were increased by > 2-fold (p < 0.05) in retina from BK-injected eyes. Eight proteins, including complement C3, were identified to be increased in both BK-stimulated rat retina and in human DME vitreous. Western blot analysis showed that Complement 3 levels in vitreous from BK-injected eyes in rats and clinical DME samples were increased by 6.6-fold (p = 0.039) and 4.3-fold (p = 0.02), compared with their respective controls. In summary, this study identifies protein changes in rat retina that are associated with BK-induced retinal thickening, including 8 proteins that were previously reported to be increased in the human DME vitreous proteome.

The G Allele of the rs12050217 Polymorphism in the BDKRB1 Gene Is Associated with Protection for Diabetic Retinopathy

Purpose: The plasma kallikrein-kinin system is activated during vascular injury caused by diabetic retinopathy (DR), being involved in hyperpermeability and inflammation. Bradykinin B1 receptor (B1R) is expressed in human retina, and its levels are increased in murine models of diabetes. Experimental studies reveal that B1R antagonists ameliorate retinal injury caused by diabetes in rodents. Thus, the aim of this study was to investigate the association between the rs12050217A/G polymorphism in the BDKRB1 gene, the gene that codifies B1R, and DR in type 2 diabetes mellitus (T2DM) patients. Methods: We analyzed 636 T2DM patients and 443 non-diabetic subjects. T2DM patients were categorized by the presence of non-proliferative DR (NPDR, n = 267), proliferative DR (PDR, n = 197), and absence of DR (n = 172). The BDKRB1 rs12050217A/G polymorphism was genotyped by real-time PCR using TaqMan MGB probes. Results: The genotype frequencies of the BDKRB1 rs12050217A/G polymorphism are in Hardy-Weinberg equilibrium and did not differ between T2DM patients and non-diabetic subjects (P > 0.05). The presence of the genotypes containing the rs12050217 G allele was less frequent in patients with PDR when compared to patients with NPDR and without DR (32.0%, 41.9%, and 43.0%, P = 0.045, respectively). Interestingly, the presence of G allele was associated with ~40% protection for PDR, which was confirmed after correction for the presence of hypertension, ethnicity, age, HDL, and gender (odds ratio = 0.616, 95% confidence interval 0.385-0.986, P = 0.043). Conclusion: For the first time, we showed that BDKRB1 rs12050217 G allele is associated with protection for the advanced stage of DR in T2DM patients; however, further studies are needed to confirm this finding.

Bradykinin potentially stimulates cell proliferation in rabbit corneal endothelial cells through the ZO‑1/ZONAB pathway

Bradykinin (BK) has been demonstrated to induce proliferation in several types of cell in ex vivo corneas. However, the mechanisms underlying the action of BK on corneal endothelial cells (CECs) remain largely unknown. The present study aimed to investigate the effect of BK on rabbit corneal endothelial cell (RCEC) proliferation, and assess the involvement of the zonula occludens‑1(ZO‑1)/ZO‑1associated nucleic acid binding protein (ZONAB) pathway. Cell proliferation and cell cycle distribution was analyzed following treatment with BK (0.01, 0.1,1.0 or 10.0 µM) for the indicated time intervals (24, 48, 72 and 96 h), or following BK treatment combined with transfection of ZONAB‑small interfering (si)RNA for 72 h. In addition, the expression of tight junction ZO‑1, nuclear ZONAB, proliferating cell nuclear antigen(PCNA) and cyclin D1 were evaluated using western blotting or immunofluorescence. BK treatment was demonstrated to induce time‑ and concentration‑dependent cell proliferation and cell cycle progression, along with the upregulation of tight junction ZO‑1 and nuclear ZONAB, as well as PCNA and cyclin D1 protein expression. Furthermore, knockdown with ZONAB‑siRNA inhibited cell proliferation, induced cell cycle arrest and downregulated PCNA and cyclin D1 protein expression. ZONAB knockdown therefore successfully reversed the increase in proliferation induced by BK treatment. Taken together, these results suggested that BK stimulated RCEC proliferation, potentially via the ZO‑1/ZONAB pathway. The signaling paradigm disclosed in the present study potentially serves as an important therapeutic target for cornea regeneration and transplantation.

Expression, distribution and function of kinin B1 receptor in the rat diabetic retina

BACKGROUND AND PURPOSE

The kinin B₁ receptor contributes to vascular inflammation and blood-retinal barrier breakdown in diabetic retinopathy (DR). We investigated the changes in expression, cellular localization and vascular inflammatory effect of B₁ receptors in retina of streptozotocin diabetic rats.

EXPERIMENTAL APPROACH

The distribution of B₁ receptors on retinal cell types was investigated by immunocytochemistry. Effects of B₁ receptor agonist, R-838, and antagonist, R-954, on retinal leukocyte adhesion, gene expression of kinin and VEGF systems, B₁ receptor immunoreactivity, microgliosis and capillary leakage were measured. Effect of B₁ receptor siRNA on gene expression was also assessed.

KEY RESULTS

mRNA levels of the kinin and VEGF systems were significantly enhanced at 2 weeks in streptozotocin (STZ)-retina compared to control-retina and were further increased at 6 weeks. B₁ receptor mRNA levels remained increased at 6 months. B₁ receptor immunolabelling was detected in vascular layers of the retina, on glial and ganglion cells. Intravitreal R-838 amplified B₁ and B₂ receptor gene expression, B₁ receptor levels (immunodetection), leukostasis and vascular permeability at 2 weeks in STZ-retina. Topical application (eye drops) of R-954 reversed these increases in B₁ receptors, leukostasis and vascular permeability. Intravitreal B₁ receptor siRNA inhibited gene expression of kinin and VEGF systems in STZ-retina. Microgliosis was unaffected by R-838 or R-954 in STZ-retina.

CONCLUSION AND IMPLICATIONS

Our results support the detrimental role of B₁ receptors on endothelial and glial cells in acute and advanced phases of DR. Topical application of the B₁ receptor antagonist R-954 seems a feasible therapeutic approach for the treatment of DR.

Oxidative stress in the optic nerve and cortical visual area of steptozotocin-induced diabetic Wistar rats: Blockade with a selective bradykinin B1 receptor antagonist

The role of bradykinin B₁ receptors on the oxidative stress as measured by the levels of Na+/K+ ATPase activity, malondialdehyde (MDA) and glutathione (GSH) in male Wistar rat optic nerve and visual cortex area 1 and 4weeks after STZ treatment was studied. Rats were divided into 4 groups (n=6-7): 1. Controls (non-diabetics); 2. Diabetics (65mg/kg streptozotocin, STZ); 3. Diabetics injected with B₁ antagonist R-954 (2mg/Kg) during the last 3days of a one week period; 4. Diabetics injected with B₁ antagonist R-954 (2mg/Kg) during the last 3days of a 4week period. The results showed that plasma glucose levels increased by up to 4 fold in diabetic rats 1 or 4weeks following the STZ treatment. R-954 treatment did significantly decrease blood glucose levels. Levels of MDA was increased in the plasma of the 1 and 4week diabetic animals whereas the GSH levels were decreased. Both markers returned to normal following R-954 treatment. Na+/K+ ATPase activity significantly decreased in the optic nerve and visual cortex of diabetic rats at 1 and 4weeks but returned to normal following R-954 treatment. MDA levels increased markedly at 1 and 4weeks compared with control levels in the optic nerve but slightly in the visual cortex and returned to control levels in both tissues following R-954 treatment. GSH levels decreased in both tissues at 1 and 4weeks compared with control levels. Following administration of the selective BKB₁R antagonist R-954, the levels of GSH returned to normal in both tissues of the 1 and 4week diabetic animals. These results showed that the inducible BKB₁ receptors are associated with the oxidative stress in the optic nerve and cortical visual area of diabetic rats and suggested that BKB₁-R antagonist R-954 could have a beneficial role in the treatment of diabetic retinopathy.

Diabetic retinopathy: Breaking the barrier

Diabetic retinopathy (DR) remains a major complication of diabetes and a leading cause of blindness among adults worldwide. DR is a progressive disease affecting both type I and type II diabetic patients at any stage of the disease, and targets the retinal microvasculature. DR results from multiple biochemical, molecular and pathophysiological changes to the retinal vasculature, which affect both microcirculatory functions and ultimately photoreceptor function. Several neural, endothelial, and support cell (e.g., pericyte) mechanisms are altered in a pathological fashion in the hyperglycemic environment during diabetes that can disturb important cell surface components in the vasculature producing the features of progressive DR pathophysiology. These include loss of the glycocalyx, blood-retinal barrier dysfunction, increased expression of inflammatory cell markers and adhesion of blood leukocytes and platelets. Included in this review is a discussion of modifications that occur at or near the surface of the retinal vascular endothelial cells, and the consequences of these alterations on the integrity of the retina.

Mechanisms of macular edema: Beyond the surface

Macular edema consists of intra- or subretinal fluid accumulation in the macular region. It occurs during the course of numerous retinal disorders and can cause severe impairment of central vision. Major causes of macular edema include diabetes, branch and central retinal vein occlusion, choroidal neovascularization, posterior uveitis, postoperative inflammation and central serous chorioretinopathy. The healthy retina is maintained in a relatively dehydrated, transparent state compatible with optimal light transmission by multiple active and passive systems. Fluid accumulation results from an imbalance between processes governing fluid entry and exit, and is driven by Starling equation when inner or outer blood-retinal barriers are disrupted. The multiple and intricate mechanisms involved in retinal hydro-ionic homeostasis, their molecular and cellular basis, and how their deregulation lead to retinal edema, are addressed in this review. Analyzing the distribution of junction proteins and water channels in the human macula, several hypotheses are raised to explain why edema forms specifically in the macular region. "Pure" clinical phenotypes of macular edema, that result presumably from a single causative mechanism, are detailed. Finally, diabetic macular edema is investigated, as a complex multifactorial pathogenic example. This comprehensive review on the current understanding of macular edema and its mechanisms opens perspectives to identify new preventive and therapeutic strategies for this sight-threatening condition.

Identification of Potential Biomarkers for Rhegmatogenous Retinal Detachment Associated with Choroidal Detachment by Vitreous iTRAQ-Based Proteomic Profiling

Rhegmatogenous retinal detachment associated with choroidal detachment (RRDCD) is a complicated and serious type of rhegmatogenous retinal detachment (RRD). In this study, we identified differentially expressed proteins in the vitreous humors of RRDCD and RRD using isobaric tags for relative and absolute quantitation (iTRAQ) combined with nano-liquid chromatography-electrospray ion trap-mass spectrometry-mass spectrometry (nano-LC-ESI-MS/MS) and bioinformatic analysis. Our result shows that 103 differentially expressed proteins, including 54 up-regulated and 49 down-regulated proteins were identified in RRDCD. Gene ontology (GO) analysis suggested that most of the differentially expressed proteins were extracellular．The Kyoto encyclopedia of genes and genomes (KEGG) pathway analysis suggested that proteins related to complement and coagulation cascades were significantly enriched. iTRAQ-based proteomic profiling reveals that complement and coagulation cascades and inflammation may play important roles in the pathogenesis of RRDCD. This study may provide novel insights into the pathogenesis of RRDCD and offer potential opportunities for the diagnosis and treatment of RRDCD.

The Role of Plasma Kallikrein-Kinin Pathway in the Development of Diabetic Retinopathy: Pathophysiology and Therapeutic Approaches

Diabetic retinal disease is characterized by a series of retinal microvascular changes and increases in retinal vascular permeability that lead to development of diabetic retinopathy (DR) and diabetic macular edema (DME), respectively. Current treatment strategies for DR and DME are mostly limited to vascular endothelial growth factor (VEGF) inhibitors and laser photocoagulation. These treatment modalities are not universally effective in all patients, and potential side effects persist in a significant portion of patients. The plasma kallikrein-kinin system (KKS) is one of the pathways that has been identified in the vitreous in proliferative DR and DME. Preclinical studies have shown that the activation of intraocular KKS induces retinal vascular permeability, vasodilation, and retinal thickening. Proteomic analysis from vitreous of eyes with DME has shown that KKS and VEGF pathways are potentially independent biologic pathways. Furthermore, proteins associated with DME in the vitreous were significantly more correlated with the KKS pathway compared to VEGF pathway. Preclinical experiments on diabetic animals showed that inhibition of KKS components was found to be an effective approach to decrease retinal vascular permeability. An initial phase I human trial of a novel plasma kallikrein inhibitor for the treatment of DME is currently ongoing to test the safety of this approach and serves as an initial step in the translation of basic science discovery into an innovative clinical intervention.

Human ciliary muscle cell responses to kinins: Activation of ERK1/2 and pro-matrix metalloproteinases secretion

AIM

To study activation of extracellular signal-regulated kinase-1/2 (ERK1/2) and pro-matrix metalloproteinases (pro-MMPs) secretion from isolated primary human ciliary muscle (h-CM) cells in response to bradykinin (BK) and other agonists.

METHODS

Serum-starved h-CM cells were challenged with vehicle, BK agonists or antagonists. Cell lysates were evaluated for phosphorylated ERK1/2 using homogeneous time-resolved fluorescence technology based on a sandwich immunoassay. Rabbit polyclonal anti-pro-MMP antibodies were used to measure pro-MMPs using immunoblot analysis.

RESULTS

A 10 min incubation time using 5 × 10⁴ h-CM cells/well was optimum condition for studying stimulation of ERK1/2 phosphorylation. BK (100 nmol/L) caused a 1.86 ± 0.26 fold (n = 3) increase in ERK1/2 phosphorylation above baseline. BK analogs, Met-Lys-BK and RMP-7 (100 nmol/L), also stimulated ERK1/2 phosphorylation by 1.57 ± 0.04 and 1.55 ± 0.09 fold, respectively. However, Des-Arg⁹-Bradykinin, a B₁ receptor-selective agonist (0.1-1 μmol/L), was essentially inactive. HOE-140 or WIN-64338 (B₂-antagonists) appreciably blocked phosphorylation of ERK1/2 induced by various BK agonists. Pre-treatment of cells with a prostaglandin (PG) synthase inhibitor (bromfenac; 1 μmol/L) failed to alter kinin-induced ERK1/2 activation. BK and a non-peptide BK agonist (FR-190997) (10 nmol/L-1 μmol/L) also enhanced pro-MMPs secretion (pro-MMP-1 > pro-MMP-3 > pro-MMP-2; 1.45-1.75-fold over baseline) from h-CM cells.

CONCLUSION

These collective data suggest that B₂ kinin receptors initiate signaling in h-CM cells by a relatively rapid mechanism (within minutes) involving ERK1/2 activation which in turn regulates MMPs production (within hours). The latter process does not involve PGs.

Plasma Kallikrein-Kinin System as a VEGF-Independent Mediator of Diabetic Macular Edema

This study characterizes the kallikrein-kinin system in vitreous from individuals with diabetic macular edema (DME) and examines mechanisms contributing to retinal thickening and retinal vascular permeability (RVP). Plasma prekallikrein (PPK) and plasma kallikrein (PKal) were increased twofold and 11.0-fold (both P < 0.0001), respectively, in vitreous from subjects with DME compared with those with a macular hole (MH). While the vascular endothelial growth factor (VEGF) level was also increased in DME vitreous, PKal and VEGF concentrations do not correlate (r = 0.266, P = 0.112). Using mass spectrometry-based proteomics, we identified 167 vitreous proteins, including 30 that were increased in DME (fourfold or more, P < 0.001 vs. MH). The majority of proteins associated with DME displayed a higher correlation with PPK than with VEGF concentrations. DME vitreous containing relatively high levels of PKal and low VEGF induced RVP when injected into the vitreous of diabetic rats, a response blocked by bradykinin receptor antagonism but not by bevacizumab. Bradykinin-induced retinal thickening in mice was not affected by blockade of VEGF receptor 2. Diabetes-induced RVP was decreased by up to 78% (P < 0.001) in Klkb1 (PPK)-deficient mice compared with wild-type controls. B2- and B1 receptor-induced RVP in diabetic mice was blocked by endothelial nitric oxide synthase (NOS) and inducible NOS deficiency, respectively. These findings implicate the PKal pathway as a VEGF-independent mediator of DME.

Enhanced Ca(2+) response and stimulation of prostaglandin release by the bradykinin B2 receptor in human retinal pigment epithelial cells primed with proinflammatory cytokines

Kallikrein, kininogen and kinin receptors are present in human ocular tissues including the retinal pigment epithelium (RPE), suggesting a possible role of bradykinin (BK) in physiological and/or pathological conditions. To test this hypothesis, kinin receptors expression and function was investigated for the first time in human fetal RPE cells, a model close to native RPE, in both control conditions and after treatment with proinflammatory cytokines. Results showed that BK evoked intracellular Ca(2+) transients in human RPE cells by activating the kinin B2 receptor. Pretreatment of the cells with TNF-α and/or IL-1β enhanced Ca(2+) response in a time- and concentration-dependent additive manner, whereas the potency of BK and that of the selective B2 receptor antagonist, fasitibant chloride, both in the nanomolar range, remained unaffected. Cytokines have no significant effect on cell number and viability and on the activity of other GPCRs such as the kinin B1, acetylcholine, ATP and thrombin receptors. Immunoblot analysis and immunofluorescence studies revealed that cytokines treatment was associated with an increase in both kinin B2 receptor and COX-2 expression and with the secretion of prostaglandin E1 and E2 into the extracellular medium. BK, through activation of the kinin B2 receptor, potentiated the COX-2 mediated prostaglandin release in cytokines-primed RPE cells while new protein synthesis and prostaglandin production contribute to the potentiating effect of cytokines on BK-induced Ca(2+) response. In conclusion, overall data revealed a cross-talk between the kinin B2 receptor and cytokines in human RPE in promoting inflammation, a key feature in retinal pathologies including diabetic retinopathy and macular edema.

Novel potential treatment modalities for ocular hypertension: focus on angiotensin and bradykinin system axes

Despite the availability of modern surgical procedures, new drug delivery techniques, health authority-approved single topical ocular drugs, and combination products thereof, there continues to be an unmet medical need for novel treatment modalities for preserving vision. This is especially true for the treatment of glaucoma and the high risk factor often associated with this ocular disease, elevated intraocular pressure (IOP). Undesirable local or systemic side effects, frequency of dosing, lack of sustained IOP lowering, and lack of prevention of diurnal IOP spikes are among the greatest challenges. The very recent discovery, characterization, and publication of 2 novel IOP-lowering agents that pertain to the renin-angiotensin and kallikrein-kinin axes potentially offer novel means to treat and control ocular hypertension (OHT). Here, some contextual introductory information is provided first, followed by more detailed discussion of the properties and actions of diminazene aceturate (DIZE; a novel angiotensin-converting enzyme-2 activator) and FR-190997 (a nonpeptide bradykinin receptor-2 agonist) in relation to their anti-OHT activities in rodent and cynomolgus monkey eyes, respectively. It is anticipated that these compounds will pave the way for future discovery, development, and marketing of novel drugs to treat glaucoma and thus help save sight for millions of people afflicted with this slow progressive optic neuropathy.

Vasoinhibins regulate the inner and outer blood-retinal barrier and limit retinal oxidative stress

Vasoinhibins are prolactin fragments present in the retina, where they have been shown to prevent the hypervasopermeability associated with diabetes. Enhanced bradykinin (BK) production contributes to the increased transport through the blood-retina barrier (BRB) in diabetes. Here, we studied if vasoinhibins regulate BRB permeability by targeting the vascular endothelium and retinal pigment epithelium (RPE) components of this barrier. Intravitreal injection of BK in male rats increased BRB permeability. Vasoinhibins prevented this effect, as did the B2 receptor antagonist Hoe-140. BK induced a transient decrease in mouse retinal and brain capillary endothelial monolayer resistance that was blocked by vasoinhibins. Both vasoinhibins and the nitric oxide (NO) synthase inhibitor L-NAME, but not the antioxidant N-acetyl cysteine (NAC), blocked the transient decrease in bovine umbilical vein endothelial cell (BUVEC) monolayer resistance induced by BK; this block was reversed by the NO donor DETANONOate. Vasoinhibins also prevented the BK-induced actin cytoskeleton redistribution, as did L-NAME. BK transiently decreased human RPE (ARPE-19) cell monolayer resistance, and this effect was blocked by vasoinhibins, L-NAME, and NAC. DETANONOate reverted the blocking effect of vasoinhibins. Similar to BK, the radical initiator Luperox induced a reduction in ARPE-19 cell monolayer resistance, which was prevented by vasoinhibins. These effects on RPE resistance coincided with actin cytoskeleton redistribution. Intravitreal injection of vasoinhibins reduced the levels of reactive oxygen species (ROS) in retinas of streptozotocin-induced diabetic rats, particularly in the RPE and capillary-containing layers. Thus, vasoinhibins reduce BRB permeability by targeting both its main inner and outer components through NO- and ROS-dependent pathways, offering potential treatment strategies against diabetic retinopathies.

Preclinical pharmacology, ocular tolerability and ocular hypotensive efficacy of a novel non-peptide bradykinin mimetic small molecule

We sought to characterize the ocular pharmacology, tolerability and intraocular pressure (IOP)-lowering efficacy of FR-190997, a non-peptidic bradykinin (BK) B2-receptor agonist. FR-190997 possessed a relatively high receptor binding affinity (Ki = 27 nM) and a high in vitro potency (EC50 = 18.3 ± 4.4 nM) for inositol-1-phosphate generation via human cloned B2-receptors expressed in host cells with mimimal activity at B1-receptors. It also mobilized intracellular Ca2+ in isolated human trabecular meshwork (h-TM), ciliary muscle (h-CM), and in immortalized non-pigmented ciliary epithelial (h-iNPE) cells (EC50s = 167-384 nM; Emax = 32-86% of BK-induced response). HOE-140, a selective B2-receptor antagonist, potently blocked the latter effects of FR-190997 (e.g., IC50 = 7.3 ± 0.6 nM in h-CM cells). FR-190997 also stimulated the release of prostaglandins (PGs) from h-TM and h-CM cells (EC50s = 60-84 nM; Emax = 29-44% relative to max. BK-induced effects). FR-190997 (0.3-300 μg t.o.) did not activate cat corneal polymodal nociceptors and did not cause ocular discomfort in Dutch-Belted rabbits, but it was not well tolerated in New Zealand albino rabbits and Hartley guinea pigs. A single topical ocular (t.o.) dose of 1% FR-190997 in Dutch-Belted rabbits and mixed breed cats did not lower IOP. However, FR-190997 efficaciously lowered IOP of conscious ocular hypertensive cynomolgus monkey eyes (e.g., 34.5 ± 7.5% decrease; 6 h post-dose of 30 μg t.o.; n = 8). Thus, FR-190997 is an unexampled efficacious ocular hypotensive B2-receptor non-peptide BK agonist that activates multiple signaling pathways to cause IOP reduction.

FR-190997, a nonpeptide bradykinin B2-receptor partial agonist, is a potent and efficacious intraocular pressure lowering agent in ocular hypertensive cynomolgus monkeys

Preclinical Research FR-190997 (8-[2,6-dichloro-3-[N-[(E)-4-(N-methylcarbamoyl) cinnaminoacetyl]-N-methylamino]benzyloxy]-2-methyl-4- (2-pyridylmethoxy) quinoline), a nonpeptide bradykinin (BK) B2-receptor-selective agonist, represents a novel class of ocular hypotensive agents. FR-190997 exhibited a high affinity for the human cloned B2-receptor (Ki = 9.8 nM) and a relatively high potency (EC50 = 155 nM) for mobilizing intracellular Ca(2+) ([Ca(2+)]i) in human ocular cells from nonpigmented ciliary epithelium; trabecular meshwork [h-TM]; ciliary muscle [h-CM] that are involved in regulating intraocular pressure (IOP). Unlike BK, FR-190997 behaved as a partial agonist (Emax = 38-80%) in these cells and its [Ca(2+)]i-mobilizing effects were blocked by the B2-receptor-selective antagonists (HOE-140, Ki = 0.8-7 nM; WIN-64338, Ki = 157-425 nM). FR-190997 stimulated the production of prostaglandins (PGs) in h-CM and h-TM cells (EC50 = 15-19 nM; Emax = 27-33%); an effect that was reduced by the cyclooxygenase-2 inhibitor bromfenac, and by HOE-140. FR-190997 also induced pro-matrix metalloproteinase (MMP)-1 and MMP-3 release from h-CM cells. FR-190997 significantly lowered IOP (37% [P < 0.001] with 30 μg, 24 h post-topical ocular dosing) in ocular hypertensive eyes of conscious Cynomolgus monkeys. This effect was reduced by bromfenac and completely blocked by a B2-antagonist. FR-190997 primarily stimulated uveoslceral outflow (UVSO) of aqueous humor (2.6 to 3.9-fold above baseline). In conclusion, FR-190997 is a B2-receptor selective partial agonist that activates phospholipase C, mobilizes [Ca(2+)]; induces PG and pro-MMP production, and that profoundly lowers IOP by promoting UVSO in ocular hypertensive Cynomolgus monkey eyes.

The kallikrein-kinin system in diabetic retinopathy

Diabetic retinopathy (DR) is a major microvascular complication associated with type 1 and type 2 diabetes mellitus, which can lead to visual impairment and blindness. Current treatment strategies for DR are mostly limited to laser therapies, steroids, and anti-VEGF agents, which are often associated with unwanted side effects leading to further complications. Recent evidence suggests that kinins play a primary role in the development of DR through enhanced vascular permeability, leukocytes infiltration, and other inflammatory mechanisms. These deleterious effects are mediated by kinin B1 and B2 receptors, which are expressed in diabetic human and rodent retina. Importantly, kinin B1 receptor is virtually absent in sane tissue, yet it is induced and upregulated in diabetic retina. These peptides belong to the kallikrein-kinin system (KKS), which contains two separate and independent pathways of regulated serine proteases, namely plasma kallikrein (PK) and tissue kallikrein (TK) that are involved in the biosynthesis of bradykinin (BK) and kallidin (Lys-BK), respectively. Hence, ocular inhibition of kallikreins or antagonism of kinin receptors offers new therapeutic avenues in the treatment and management of DR. Herein, we present an overview of the principal features and known inflammatory mechanisms associated with DR along with the current therapeutic approaches and put special emphasis on the KKS as a new and promising therapeutic target due to its link with key pathways directly associated with the development of DR.

Plasma kallikrein-kinin system and diabetic retinopathy

Diabetic retinopathy (DR) occurs, to some extent, in most people with at least 20 years' duration of diabetes mellitus. The progression of DR to its sight-threatening stages is usually associated with the worsening of underlying retinal vascular dysfunction and disease. The plasma kallikrein-kinin system (KKS) is activated during vascular injury, where it mediates important functions in innate inflammation, blood flow, and coagulation. Recent findings from human vitreous proteomics and experimental studies on diabetic animal models have implicated the KKS in contributing to DR. Vitreous fluid from people with advanced stages of DR contains increased levels of plasma KKS components, including plasma kallikrein (PK), coagulation factor XII, and high-molecular-weight kininogen. Both bradykinin B1 and B2 receptor isoforms (B1R and B2R, respectively) are expressed in human retina, and retinal B1R levels are increased in diabetic rodents. The activation of the intraocular KKS induces retinal vascular permeability, vasodilation, and retinal thickening, and these responses are exacerbated in diabetic rats. Preclinical studies have shown that the administration of PK inhibitors and B1R antagonists to diabetic rats ameliorates retinal vascular hyperpermeability and inflammation. These findings suggest that components of plasma KKS are potential therapeutic targets for diabetic macular edema.

Temporal development of retinal arteriolar endothelial dysfunction in porcine type 1 diabetes

PURPOSE

Although hyperglycemia is implicated in retinal vascular dysfunction associated with the development of diabetic retinopathy, the temporal influence of hyperglycemia on retinal arteriolar reactivity remains unclear. Development of a large animal model of diabetes relevant to the human retina for evaluation of vascular function is also lacking. Herein, we examined nitric oxide (NO)-mediated dilation and endothelin-1 (ET-1)-induced constriction in retinal arterioles at various time periods in a porcine model of type 1 diabetes.

METHODS

Retinal arterioles were isolated from streptozocin-induced diabetic pigs (2, 6, and 12 weeks of hyperglycemia, 427 ± 23 mg/dL) and age-matched control pigs (73 ± 4 mg/dL), and then cannulated and pressurized for vasoreactivity study using videomicroscopic techniques.

RESULTS

Retinal arterioles isolated from control and diabetic pigs developed comparable levels of myogenic tone. The endothelium-dependent NO-mediated vasodilations to bradykinin and stepwise increases in luminal flow were significantly reduced within 2 weeks of hyperglycemia. The inhibitory effect was comparable following 6 and 12 weeks of hyperglycemia. However, the endothelium-independent vasodilation to sodium nitroprusside was unaffected. Constriction of retinal arterioles to ET-1 was unaltered at all time periods of hyperglycemia.

CONCLUSIONS

Our findings provide the first direct evidence for selective impairment of endothelium-dependent NO-mediated dilation of retinal arterioles within 2 weeks of hyperglycemia in a pig model of diabetes. By contrast, the ability of arteriolar smooth muscle to dilate to NO donor or contract to ET-1 was unaffected throughout the study period. This endothelial vasodilator dysfunction during early diabetes may contribute to development of retinopathy with chronic hyperglycemia.

Kallistatin in blood pressure regulation transgenic and somatic gene delivery studies

Kallistatin, first discovered as a human kallikrein-binding protein in the circulation, shares high homology with other plasma serine proteinase inhibitors (serpins). It forms a covalently linked complex with tissue kallikrein and inhibits kallikrein's activity. Substantial evidence has accumulated in recent years indicating that kallistatin may play a role in blood pressure regulation independent of its interaction with tissue kallikrein. Intravenous injection of kallistatin into rats and mice results in a rapid and transient reduction of blood pressure in a dose-dependent manner. Functional analysis in transgenic mice over-expressing rat kallikrein-binding protein, an analogue of human kallistatin, revealed that these mice have significantly lower blood pressure compared with control littermates. Adenovirus-mediated delivery of the human kallistatin gene can cause significant blood pressure reductions for 4 weeks in spontaneously hypertensive rats. Finally, kallistatin can induce vasorelaxation in isolated rat aortic rings and reduce renal perfusion pressure in the isolated, perfused kidney. Together, these findings suggest a direct role for kallistatin in regulating blood pressure and raise the possibility for the development of new pharmacological treatments for hypertension. (Trends Cardiovasc Med 1997;7:307-311). © 1997, Elsevier Science Inc.

Alternative pathways in the development of diabetic retinopathy: the renin-angiotensin and kallikrein-kinin systems

Diabetic retinopathy is a common complication of both type 1 and type 2 diabetes and is the leading cause of blindness in people of working age. Current treatment strategies are mostly limited to laser photocoagulation, which restricts proliferative retinopathic changes but also causes irreversible damage to the retina. This review examines two important pathways involved in regulating vascular function and their role in the development of diabetic retinopathy. One, the renin-angiotensin system, is well known and has established angiogenic effects on the retina that increase in diabetic retinopathy. The other, the kallikrein-kinin system, has recently been found to be important in the development of diabetic retinal complications. This review describes the components of the two signalling networks, examines the current animal model studies investigating the role of these pathways in diabetic retinopathy and reviews the clinical studies that have been undertaken examining systemic inhibition of different points in these pathways. These systems are promising targets for therapies aimed at inhibiting the development of diabetic retinopathy and in the future, combination therapies that take advantage of both pathways might result in new treatment options for this debilitating complication of diabetes.

Retinal arteriolar responses to acute severe elevation in systemic blood pressure in cats: role of endothelium-derived factors

The purpose of this study was to investigate the roles of endothelium-derived factors in the retinal arteriolar responses to acute severe elevation in systemic blood pressure (BP) in cats. Acute elevation of mean arterial BP by 60% for 5 min was achieved by inflating a balloon-tipped catheter in the descending aorta. The retinal arteriolar diameter, flow velocity, wall shear rate (WSR) and blood flow (RBF) changes during BP elevation were assessed with laser Doppler velocimetry 2 h after intravitreal injections of nitric oxide (NO) synthase inhibitor l-NAME, cyclooxygenase inhibitor indomethacin, endothelin-1 receptor antagonists (BQ-123 for type A and BQ-788 for type B), or Rho kinase inhibitor fasudil. BP elevation caused a marked increase in retinal arteriolar flow velocity and WSR with slight vasoconstriction, resulting in an increase in RBF. The increases in velocity, WSR and RBF, but not diameter, were correlated with the increase in ocular perfusion pressure. With l-NAME or indomethacin, the increase in RBF upon BP elevation was significantly attenuated due to enhanced retinal arteriolar vasoconstriction. In contrast, BQ-123 and fasudil potentiated the increased RBF. BQ-788 had no effect on arteriolar diameter and hemodynamics. Our data suggest that acute elevation of BP by 60% leads to an increase in RBF due to the release of NO and prostanoids probably through a shear stress-induced vasodilation mechanism. The release of endothelin-1 and Rho kinase activation help to limit RBF augmentation by counteracting the vasodilation. It appears that the retinal endothelium, by releasing vasoactive substances, contributes to RBF regulation during acute severe elevation of systemic blood pressure.

Blockade of early and late retinal biochemical alterations associated with diabetes development by the selective bradykinin B1 receptor antagonist R-954

The chronic hyperglycemia measured alongside diabetes development is associated with significant long-term damage and failure of various organs. In the present study it was shown that hyperglycemia induced early and long term increases in nitric oxide (NO) levels, kallikrein activity and vascular capillary permeability measured as plasma extravasation, and decreases of Na/K ATPase activity in diabetic rat retina 4 and 12 weeks after streptozotocin (STZ) injection. Treatment of the animals for 5 consecutive days with a novel selective bradykinin B(1) receptor (BKB(1)-R) antagonist R-954 (2mg/kg s.c) at the end of the 4 and 12 week periods highly reduced NO, kallikrein and capillary permeability and increased Na/K ATPase activity in the retina. These results suggest that the BKB(1)-R receptor subtype is over-expressed during the streptozotocin-induced development of diabetes in rat retina as evidenced by the inhibitory effects of the BKB(1)-R antagonist R-954 on NO, kallikrein and vascular permeability increases as well as Na/K ATPase decreases. The beneficial role of the BKB(1)-R antagonist R-954 for the treatment of the diabetic retinopathy is also suggested.

Ocular application of the kinin B1 receptor antagonist LF22-0542 inhibits retinal inflammation and oxidative stress in streptozotocin-diabetic rats

PURPOSE

Kinin B(1) receptor (B(1)R) is upregulated in retina of Streptozotocin (STZ)-diabetic rats and contributes to vasodilation of retinal microvessels and breakdown of the blood-retinal barrier. Systemic treatment with B(1)R antagonists reversed the increased retinal plasma extravasation in STZ rats. The present study aims at determining whether ocular application of a water soluble B(1)R antagonist could reverse diabetes-induced retinal inflammation and oxidative stress.

METHODS

Wistar rats were made diabetic with STZ (65 mg/kg, i.p.) and 7 days later, they received one eye drop application of LF22-0542 (1% in saline) twice a day for a 7 day-period. The impact was determined on retinal vascular permeability (Evans blue exudation), leukostasis (leukocyte infiltration using Fluorescein-isothiocyanate (FITC)-coupled Concanavalin A lectin), retinal mRNA levels (by qRT-PCR) of inflammatory (B(1)R, iNOS, COX-2, ICAM-1, VEGF-A, VEGF receptor type 2, IL-1β and HIF-1α) and anti-inflammatory (B(2)R, eNOS) markers and retinal level of superoxide anion (dihydroethidium staining).

RESULTS

Retinal plasma extravasation, leukostasis and mRNA levels of B(1)R, iNOS, COX-2, VEGF receptor type 2, IL-1β and HIF-1α were significantly increased in diabetic retinae compared to control rats. All these abnormalities were reversed to control values in diabetic rats treated with LF22-0542. B(1)R antagonist also significantly inhibited the increased production of superoxide anion in diabetic retinae.

CONCLUSION

B(1)R displays a pathological role in the early stage of diabetes by increasing oxidative stress and pro-inflammatory mediators involved in retinal vascular alterations. Hence, topical application of kinin B(1)R antagonist appears a highly promising novel approach for the treatment of diabetic retinopathy.

Bradykinin-mediated cell proliferation depends on transactivation of EGF receptor in corneal fibroblasts

In previous studies, bradykinin (BK) has been shown to induce cell proliferation through BK B2 receptor (B2R) via p42/p44 MAPK in Statens Seruminstitut Rabbit Corneal Cells (SIRCs). In addition to this pathway, EGFR transactivation pathway has been implicated in linking a variety of G-protein coupled receptors to MAPK cascades. Here, we further investigate whether these transactivation mechanisms participating in BK-induced cell proliferation in SIRCs. Using an immunofluorescence staining and RT-PCR, we initially characterize that SIRCs were corneal fibroblasts and predominantly expressed B2R by BK. Inhibition of p42/p44 MAPK by the inhibitors of Src, EGFR, and Akt or transfection with respective siRNAs prevents BK-induced DNA synthesis in SIRCs. The mechanisms underlying these responses were mediated through phosphorylation of Src and EGFR via the formation of Src/EGFR complex which was attenuated by PP1 and AG1478. Moreover, BK-induced p42/p44 MAPK and Akt activation was mediated through EGFR transactivation, which was diminished by the inhibitors of MMP-2/9 and heparin-binding EGF-like factor (HB-EGF). Finally, increased nuclear translocation of Akt and p42/p44 MAPK turns on early gene expression leading to cell proliferation. These results suggest that BK-induced cell proliferation is mediated through c-Src-dependent transactivation of EGFR via MMP2/9-dependent pro-HB-EGF shedding linking to activation of Akt and p42/p44 MAPK in corneal fibroblasts.

High molecular weight kininogen activates B2 receptor signaling pathway in human vascular endothelial cells

The nonenzymatic cofactor high molecular weight kininogen (HK) is a precursor of bradykinin (BK). The production of BK from HK by plasma kallikrein has been implicated in the pathogenesis of inflammation and vascular injury. However, the functional role of HK in the absence of prekallikrein (PK), the proenzyme of plasma kallikrein, on vascular endothelial cells is not fully defined. In addition, no clinical abnormality is seen in PK-deficient patients. Therefore, an investigation into the effect of HK, in the absence of PK, on human pulmonary artery endothelial cell (HPAEC) function was performed. HK caused a marked and dose-dependent increase in the intracellular calcium [Ca(2+)](i) level in HPAEC. Gd(3+) and verapamil potentiated the HK-induced increase in [Ca(2+)](i). HK-induced Ca(2+) increase stimulated endothelial nitric oxide (NO) and prostacyclin (PGI(2)) production. The inhibitors of B(2) receptor-dependent signaling pathway impaired HK-mediated signal transduction in HPAEC. HK had no effect on endothelial permeability at physiological concentration. This study demonstrated that HK regulates endothelial cell function. HK could play an important role in maintaining normal endothelial function and blood flow and serve as a cardioprotective peptide.

Plasma kallikrein mediates retinal vascular dysfunction and induces retinal thickening in diabetic rats

OBJECTIVE

Plasma kallikrein (PK) has been identified in vitreous fluid obtained from individuals with diabetic retinopathy and has been implicated in contributing to retinal vascular dysfunction. In this report, we examined the effects of PK on retinal vascular functions and thickness in diabetic rats.

RESEARCH DESIGN AND METHODS

We investigated the effects of a selective PK inhibitor, ASP-440, and C1 inhibitor (C1-INH), the primary physiological inhibitor of PK, on retinal vascular permeability (RVP) and hemodynamics in rats with streptozotocin-induced diabetes. The effect of intravitreal PK injection on retinal thickness was examined by spectral domain optical coherence tomography.

RESULTS

Systemic continuous administration of ASP-440 for 4 weeks initiated at the time of diabetes onset inhibited RVP by 42% (P = 0.013) and 83% (P < 0.001) at doses of 0.25 and 0.6 mg/kg per day, respectively. Administration of ASP-440 initiated 2 weeks after the onset of diabetes ameliorated both RVP and retinal blood flow abnormalities in diabetic rats measured at 4 weeks' diabetes duration. Intravitreal injection of C1-INH similarly decreased impaired RVP in rats with 2 weeks' diabetes duration. Intravitreal injection of PK increased both acute RVP and sustained focal RVP (24 h postinjection) to a greater extent in diabetic rats compared with nondiabetic control rats. Intravitreal injection of PK increased retinal thickness compared with baseline to a greater extent (P = 0.017) in diabetic rats (from 193 ± 10 μm to 223 ± 13 μm) compared with nondiabetic rats (from 182 ± 8 μm to 193 ± 9 μm).

CONCLUSIONS

These results show that PK contributes to retinal vascular dysfunctions in diabetic rats and that the combination of diabetes and intravitreal injection of PK in rats induces retinal thickening.

Bradykinin activation of extracellular signal-regulated kinases in human trabecular meshwork cells

Bradykinin stimulation of B(2) kinin receptors has been shown to promote matrix metallo-proteinase (MMP) secretion from trabecular meshwork cells and to increase conventional outflow facility. Because acute secretion of MMPs can be dependent on the activity of extracellular signal-regulated MAP kinases (ERK1/2), experiments were performed to determine bradykinin effects on ERK1/2 in cultured human trabecular meshwork cells and the relationship of these effects to MMP-9 release. Treatment of cells with bradykinin produced a rapid 4-to 6-fold increase in ERK1/2 phosphorylation. Stimulation of ERK1/2 activity peaked within 2 min and then declined to control levels by 60 min. The response maximum occurred with 100nM bradykinin and the estimated EC₅₀ was 0.7nM. Treatment of cells with the B₂ kinin receptor agonist, Tyr⁸- bradykinin, also stimulated ERK1/2 phosphorylation while the B₁ agonist, Lys- [Des-Arg⁹]- bradykinin had no significant effect. In addition, activation of ERK1/2 by bradykinin or Tyr⁸- bradykinin was blocked by the selective B₂ receptor antagonist, Hoe-140. Inhibition of MAP kinase kinase (MEK) with U0126 also blocked bradykinin-induced ERK1/2 phosphorylation. Suppression of protein kinase C activity with the nonselective inhibitor, GF109203X, or by down-regulation with phorbol ester, diminished, but did not eliminate, bradykinin activation of ERK1/2. A similar decrease of ERK1/2 stimulation was observed when Src kinase was inhibited by 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP2). Finally, blockade of bradykinin-induced ERK1/2 activation substantially reduced the peptide's action to stimulate MMP-9 release into the extracellular environment. The data demonstrate that bradykinin promotes ERK1/2 activation in human trabecular meshwork cells. The effect is mediated by B₂ kinin receptors, involves two different signaling pathways, and results in increased secretion of MMP-9.

Modulation of retinal blood flow by kinin B₁ receptor in Streptozotocin-diabetic rats

The vasoactive kinin B₁ receptor (B₁R) is overexpressed in the retina of diabetic rats in response to hyperglycemia and oxidative stress. The aim of the present study was to determine whether B₁R could contribute to the early retinal blood flow changes occurring in diabetes. Male Wistar rats were rendered diabetic with a single i.p. injection of Streptozotocin (STZ) and studied 4 days or 6 weeks after diabetes induction. The presence of B₁R in the retina was confirmed by Western blot. The impact of oral administration of the B₁R selective antagonist SSR240612 (10mg/kg) was measured on alteration of retinal perfusion in awake diabetic rats by quantitative autoradiography. Data showed that B₁R was upregulated in the STZ-diabetic retina at 4 days and 6 weeks. Retinal blood flow was not altered in 4-day diabetic rats compared with age-matched controls but was significantly decreased following SSR240612 treatment. In 6-week diabetic rats, retinal blood flow was markedly reduced compared to control rats and SSR240612 did not further decrease the blood flow. These results suggest that B₁R is upregulated in STZ-diabetic retina and has a protective compensatory role on retinal microcirculation at 4 days but not at 6 weeks following diabetes induction.

Plasma kallikrein and diabetic macular edema

Recent proteomic studies have identified components of the kallikrein kinin system, including plasma kallikrein, factor XII, and kininogen, in vitreous obtained from individuals with advanced diabetic retinopathy. In rodent models, activation of plasma kallikrein in vitreous increases retinal vascular permeability; whereas inhibition of the kallikrein kinin system reduces retinal leakage induced by diabetes and hypertension. These findings suggest that intraocular activation of the plasma kallikrein pathway may contribute to excessive retinal vascular permeability that can lead to diabetic macular edema. The kallikrein kinin system contains two separate and independently regulated serine proteases that generate bradykinin peptides: plasma kallikrein and tissue kallikrein. Tissue kallikrein is expressed in the retina and ciliary body, where it has been implicated in exerting autocrine or paracrine effects via bradykinin receptors that are colocalized in these tissues. Emerging evidence suggests that plasma kallikrein inhibitors may provide a new therapeutic opportunity to reduce retinal vascular permeability.

The renin-angiotensin system in retinal health and disease: Its influence on neurons, glia and the vasculature

Renin-Angiotensin System is classically recognized for its role in the control of systemic blood pressure. However, the retina is recognized to have all the components necessary for angiotensin II formation, suggestive of a role for Angiotensin II in the retina that is independent of the systemic circulation. The most well described effects of Angiotensin II are on the retinal vasculature, with roles in vasoconstriction and angiogenesis. However, it is now emerging that Angiotensin II has roles in modulation of retinal function, possibly in regulating GABAergic amacrine cells. In addition, Angiotensin II is likely to have effects on glia. Angiotensin II has also been implicated in retinal vascular diseases such as Retinopathy of Prematurity and diabetic retinopathty, and more recently actions in choroidal neovascularizaiton and glaucoma have also emerged. The mechanisms by which Angiotensin II promotes angiogensis in retinal vascular diseases is indicative of the complexity of the RAS and the variety of cell types that it effects. Indeed, these diseases are not purely characterized by direct effects of Angiotensin II on the vasculature. In retinopathy of prematurity, for example, blockade of AT1 receptors prevents pathological angiogenesis, but also promotes revascularization of avascular regions of the retina. The primary site of action of Angiotensin II in this disease may be on retinal glia, rather than the vasculature. Indeed, blockade of AT1 receptors prevents glial loss and promotes the re-establishment of normal vessel growth. Blockade of RAS as a treatment for preventing the incidence and progression of diabetic retinopathy has also emerged based on a series of studies in animal models showing that blockade of the RAS prevents the development of a variety of vascular and neuronal deficits in this disease. Importantly these effects may be independent of actions on systemic blood pressure. This has culminated recently with the completion of several large multi-centre clinical trials that showed that blockade of the RAS may be of benefit in some at risk patients with diabetes. With the emergence of novel compounds targeting different aspects of the RAS even more effective ways of blocking the RAS may be possible in the future.

Peptide hormone regulation of angiogenesis

It is now apparent that regulation of blood vessel growth contributes to the classical actions of hormones on development, growth, and reproduction. Endothelial cells are ideally positioned to respond to hormones, which act in concert with locally produced chemical mediators to regulate their growth, motility, function, and survival. Hormones affect angiogenesis either directly through actions on endothelial cells or indirectly by regulating proangiogenic factors like vascular endothelial growth factor. Importantly, the local microenvironment of endothelial cells can determine the outcome of hormone action on angiogenesis. Members of the growth hormone/prolactin/placental lactogen, the renin-angiotensin, and the kallikrein-kinin systems that exert stimulatory effects on angiogenesis can acquire antiangiogenic properties after undergoing proteolytic cleavage. In view of the opposing effects of hormonal fragments and precursor molecules, the regulation of the proteases responsible for specific protein cleavage represents an efficient mechanism for balancing angiogenesis. This review presents an overview of the actions on angiogenesis of the above-mentioned peptide hormonal families and addresses how specific proteolysis alters the final outcome of these actions in the context of health and disease.

Divergent roles of nitric oxide and rho kinase in vasomotor regulation of human retinal arterioles

PURPOSE

Although the arteriolar segment contributes to flow regulation, there is sparse information at the single microvessel level on how vasomotor function is regulated in the human retina. The authors have previously reported vasoreactivity and its underlying mechanisms in isolated porcine retinal arterioles. Herein, they studied human retinal arterioles for comparison.

METHODS

Retinal tissues were obtained from seven patients undergoing enucleation. Human and porcine retinal arterioles were isolated and pressurized to 55 cm H(2)O luminal pressure for vasoreactivity study using videomicroscopic techniques.

RESULTS

Isolated human and porcine retinal arterioles developed myogenic tone and dilated dose dependently to bradykinin, adenosine, and sodium nitroprusside. Stepwise increases in luminal flow produced graded dilation with approximately 60% dilation at the highest flow tested. Nitric oxide (NO) synthase inhibitor L-NAME nearly abolished dilations to bradykinin and flow and attenuated the adenosine-induced dilation without altering the response to nitroprusside. Endothelin-1 caused dose-dependent constriction. Rho kinase (ROCK) inhibitor H-1152 blocked both myogenic tone and endothelin-1-induced constriction. Responses of retinal arterioles to all agonists and increased flow were similar between pigs and humans.

CONCLUSIONS

Isolated human retinal arterioles dilate to bradykinin and increased flow in an NO-dependent manner. NO contributes, in part, to adenosine-induced vasodilation. Conversely, ROCK activation mediates myogenic tone and endothelin-1-induced vasoconstriction. Similarities in these vasoactive responses and the underlying mechanisms between human and porcine retinal arterioles support the latter as a viable experimental model of the human retinal microcirculation.

Localization of complement 1 inhibitor (C1INH/SERPING1) in human eyes with age-related macular degeneration

Age-related macular degeneration (AMD) is a common degenerative disease resulting in injury to the retina, retinal pigment epithelium and choriocapillaris. Recent data from histopathology, animal models and genetic studies have implicated altered regulation of the complement system as a major factor in the incidence and progression of this disease. A variant in the gene SERPING1, which encodes C1INH, an inhibitor of the classical and lectin pathways of complement activation, was recently shown to be associated with AMD. In this study we sought to determine the localization of C1INH in human donor eyes. Immunofluorescence studies using a monoclonal antibody directed against C1INH revealed localization to photoreceptor cells, inner nuclear layer neurons, choriocapillaris, and choroidal extracellular matrix. Drusen did not exhibit labeling. Genotype at rs2511989 did not appear to affect C1INH abundance or localization, nor was it associated with significant molecular weight differences when evaluated by Western blot. In a small number of eyes (n = 7 AMD and n = 7 control) AMD affection status was correlated with increased abundance of choroidal C1INH. These results indicate that C1INH protein is present in the retina and choroid, where it may regulate complement activation.

Molecular Analysis of Neurolysin Expression in the Rat and Bovine Ciliary Body

PURPOSE

This paper deals with the capability of the ciliary epithelium to express neurolysin, involved in the inactivation of numerous neuropeptides.

METHODS

Total RNAs from ciliary body (CB) were processed for RT-PCR, and the amplification products were sequenced. The whole-protein extracts of CBs were analyzed using the Western blot. The CBs were processed for neurolysin immunolocalization.

RESULTS

The RT-PCR detected the presence of neurolysin mRNA in the ciliary body. The Western blot assays demonstrated immunochemical cross-reactivity with neurolysin. The immunoreactivity to neurolysin was observed in ciliary epithelium.

CONCLUSIONS

These results indicate that the ciliary epithelium expresses neurolysin.

Expression of the kallikrein/kinin system in human anterior segment

Tissue kallikrein acts on the substrate, low molecular weight kininogen, to liberate bradykinin in a variety of tissues. Bradykinin stimulation of B(2) kinin receptors has been shown to initiate signaling in trabecular meshwork cells and increase conventional outflow facility. The objective of the present study was to determine if the components for kinin generation and response are expressed in tissues of the human anterior segment. Expression of mRNA encoding tissue kallikrein (KK), low molecular weight kininogen, and B(1) and B(2) kinin receptors was examined in human ciliary smooth muscle (CM), trabecular meshwork (TM) and non-pigmented epithelial (NPE) cells using RT-PCR. Expression of component proteins was also investigated by immunohistochemical analyses performed on parasagittal sections of human anterior segment and TM cells, and by immunoblot. KK mRNA was detected in NPE cells and in cultured CM and TM cells from multiple donors. Each cell type also expressed mRNAs encoding both B(1) and B(2) kinin receptors. Immunohistochemical analysis of KK protein in sectioned anterior segment supported the RT-PCR results. Intense KK immunofluorescence was observed in the epithelial lining of the ciliary body and KK protein was also detected in the ciliary muscle. KK protein expression within the TM was demonstrated by analyses of TM tissue and cultured TM cells. The presence of KK along with B(1) and B(2) receptor proteins was confirmed by immunoblots of cell lysates prepared from CM, NPE or TM cells. Finally, both CM and TM cells were found to possess enzymes for bradykinin inactivation. These data demonstrate that key components for kinin generation and regulation are localized within the human anterior segment. Further, multiple cell types express both B(1) and B(2) kinin receptors and are targets for kinin action. The results support the possibility that kinins produced within the eye may contribute to the regulation of aqueous outflow.

Bradykinin stimulates glutamate uptake via both B1R and B2R activation in a human retinal pigment epithelial cells

AIMS

We were to examine the effect of bradykinin (BK) in the regulation of glutamate transporter and its related signaling molecules in a human retinal pigment epithelial (ARPE) cells, which are important cells to support retina.

MAIN METHODS

d-[2,3-(3)H]-aspartate uptake, western immunoblotting, reverse transcription polymerase chain reaction, [(3)H]-arachidonic acid release, and siRNA transfection techniques were used.

KEY FINDINGS

BK stimulated glutamate uptake as well as the mRNA expression of excitatory amino acid transporter 4 (EAAT4) and excitatory amino acid carrier 1 (EAAC1), which was blocked by treatment with bradykinin 1 receptor (B1R) and bradykinin 2 receptor (B2R) siRNA, suggesting the role of B1R and B2R in this process. The BK-induced stimulation of glutamate uptake was also blocked by [des-Arg(10)]-HOE 140, a B1R antagonist, and HOE 140, a B2R antagonist, as well as by the tyrosine kinase inhibitors genistein and herbimycin A. In addition, the BK-induced stimulation of glutamate uptake was blocked by treatment with the phospholipase A(2) inhibitors mepacrine and AACOCF(3), the cyclooxygenase (COX) inhibitor indomethacin, and the COX-2 inhibitor Dup 697. Furthermore, the BK-induced increase in COX-2 expression was blocked by the PI-3 kinase inhibitors wortmannin and LY294002, Akt inhibitor, and the protein kinase C (PKC) inhibitors staurosporine and bisindolylmaleimide I, suggesting the role of PI-3 kinase and PKC in this process. BK stimulated Akt activation and the translocation of PKC activation via the activation of B1R and B2R.

SIGNIFICANCE

BK stimulates glutamate uptake through a PKC-Akt-COX-2 signaling cascade in ARPE cells.

Retinal plasma extravasation in streptozotocin-diabetic rats mediated by kinin B(1) and B(2) receptors

BACKGROUND AND PURPOSE

We investigated whether or not kinin receptors play a role in diabetic blood-retinal barrier breakdown, which is a leading cause of vision loss.

EXPERIMENTAL APPROACH

Blood-retinal barrier breakdown was quantified using Evans blue, and expression of kinin B(1) receptor mRNA was measured using quantitative reverse transcrition-PCR. Diabetic rats (streptozotocin (STZ), 65 mg kg(-1)) received a single intraocular injection of bradykinin (BK) or des-Arg(9)-BK, alone, or in combination with antagonists for B(1) (des-Arg(10)-Hoe140, R-715) and/or B(2) (Hoe140) receptors, given intraocularly or intravenously (i.v.).

KEY RESULTS

In control rats, BK (0.1-10 nmol) dose-dependently increased plasma extravasation, which was inhibited by Hoe140 (0.2 nmol), whereas des-Arg(9)-BK (0.1 and 1 nmol) was without effect. B(1) receptor mRNA was markedly increased in retinas of diabetic rats, and this was prevented by N-acetyl-L-cysteine (1 g kg(-1) day(-1) for 7 days). Plasma extravasation in retinas of STZ-diabetic rats was higher than in controls and enhanced by des-Arg(9)-BK. Response to des-Arg(9)-BK was inhibited by intraocular or i.v. injection of B(1) receptor antagonists. Diabetes-induced plasma extravasation was inhibited only by a combination of des-Arg(10)-Hoe140 and Hoe 140 (100 nmol kg(-1), i.v. 15 min earlier) or by R-715 (1 micromol kg(-1), i.v.) injected daily for 7 days.

CONCLUSIONS AND IMPLICATIONS

Kinin B(1) receptors are upregulated in retinas of STZ-diabetic rats through a mechanism involving oxidative stress. Both kinin B(1) and B(2) receptors contribute to increased plasma extravasation in diabetic retinopathy. Chronic inhibition of both kinin receptors, possibly with antioxidant adjuvants, may be a novel therapeutic strategy for diabetic retinopathy.