Hypertension is a leading risk factor for the development and progression of diabetic retinopathy and contributes to a variety of other retinal diseases in the absence of diabetes mellitus. Inhibition of the renin-angiotensin system has been shown to provide beneficial effects against diabetic retinopathy, both in the absence and presence of hypertension, suggesting that angiotensin II (Ang II) and the Ang II type 1 receptor may contribute to retinal vascular dysfunction. We investigated the effects of the Ang II type 1 receptor antagonist candesartan on retinal vascular permeability (RVP) in normotensive rats with streptozotocin-induced diabetes mellitus and in rats with Ang II-induced hypertension. We showed that candesartan treatment decreased diabetes mellitus- and Ang II-stimulated RVP by 58% (P<0.05) and 79% (P<0.05), respectively, compared with untreated controls, suggesting that activation of the Ang II type 1 receptor contributes to blood-retinal barrier dysfunction. We found that plasma kallikrein levels are increased in the retina of rats with Ang II-stimulated hypertension and that intravitreal injection of either plasma kallikrein or bradykinin is sufficient to increase RVP. We showed that a novel small molecule inhibitor of plasma kallikrein, 1-benzyl-1H-pyrazole-4-carboxylic acid 4-carbamimidoyl-benzylamide, delivered systemically via a subcutaneous pump, decreased Ang II-stimulated RVP by 70% (P<0.05) and ameliorates Ang II-induced hypertension, measured from the carotid artery by telemetry, but did not reduce Ang II-induced retinal leukostasis. These findings demonstrate that activation of the Ang II type 1 receptor increases RVP and suggest that systemic plasma kallikrein inhibition may provide a new therapeutic approach for ameliorating blood-retinal barrier dysfunction induced by hypertension.

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.

Plasma kallikrein is a serine protease and circulating component of inflammation, which exerts clinically significant effects on vasogenic edema. This study examines the role of plasma kallikrein in VEGF-induced retinal edema.

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.

Hyperglycemia is associated with greater hematoma expansion and poor clinical outcomes after intracerebral hemorrhage. We show that cerebral hematoma expansion triggered by intracerebral infusion of autologous blood is greater in diabetic rats and mice compared to nondiabetic controls and that this augmented expansion is ameliorated by plasma kallikrein (PK) inhibition or deficiency. Intracerebral injection of purified PK augmented hematoma expansion in both diabetic and acutely hyperglycemic rats, whereas injection of bradykinin, plasmin or tissue plasminogen activator did not elicit such a response. This response, which occurs rapidly, was prevented by co-injection of the glycoprotein VI agonist convulxin and was mimicked by glycoprotein VI inhibition or deficiency, implicating an effect of PK on inhibiting platelet aggregation. We show that PK inhibits collagen-induced platelet aggregation by binding collagen, a response enhanced by elevated glucose concentrations. The effect of hyperglycemia on hematoma expansion and PK-mediated inhibition of platelet aggregation could be mimicked by infusing mannitol. These findings suggest that hyperglycemia augments cerebral hematoma expansion by PK-mediated osmotic-sensitive inhibition of hemostasis.

Leukocyte adhesion in retinal microvasuculature substantially contributes to diabetic retinopathy. Involvement of the Rho/Rho kinase (ROCK) pathway in diabetic microvasculopathy and therapeutic potential of fasudil, a selective ROCK inhibitor, are investigated.

Dexamethasone, a synthetic corticosteroid, is widely used as a potent anti-inflammatory drug in various diseases including corneal angiogenesis. However, dexamethasone's impact on interleukin (IL)-1beta-dependent inflammatory angiogenesis is unknown. Here, we show that dexamethasone inhibits IL-1beta-induced neovascularization and the expression of the angiogenesis-related factors, vascular endothelial growth factor-A, KC, and prostaglandin E(2) in the mouse cornea 2 days after IL-1beta implantation. IL-1beta caused IkappaB-alpha phosphorylation in corneal stromal cells but not in infiltrated CD11b(+) cells 2 days after IL-1beta implantation. In contrast, both cell types were positive for phosphorylated IkappaB-alpha 4 days after IL-1beta implantation. Dexamethasone significantly inhibited IkappaB-alpha phosphorylation 2 and 4 days after IL-1beta implantation. Furthermore, dexamethasone inhibited IL-1beta-induced expression of vascular endothelial growth factor-A, KC, and prostaglandin E(2), and signaling of nuclear factor (NF)-kappaB in corneal fibroblasts in vitro. A selective NF-kappaB inhibitor attenuated IL-1beta-induced corneal angiogenesis. These findings suggest that NF-kappaB activation in the corneal stromal cells is an important early event during IL-1beta-induced corneal angiogenesis and that dexamethasone inhibits IL-1beta-induced angiogenesis partially via blocking NF-kappaB signaling.