Kallikrein-Kinin System: An Emerging Competitor or Collaborator for VEGF in Diabetic Macular Edema?

An increasing number of patients with diabetes suffer from vision-threatening diabetic retinopathy (DR), i.e., proliferative diabetic retinopathy (PDR) and diabetic macular edema (DME), worldwide (1). Vascular endothelial growth factor (VEGF) plays a key role in the angiogenic responses in PDR, and the development of anti-VEGF therapy has reduced the burdens of PDR patients (2,3). Retinal vascular permeability leads to morphological and functional damages in the neuroglial components in the retinas and concomitant visual disturbance in DME (4–6). Although anti-VEGF agents have been effective for many patients with DME, their effects are often slow and partial and the benefits are limited, suggesting that there are other cellular and molecular mechanisms in addition to VEGF (7). Diabetes-induced disruption of the blood-retinal barrier (BRB) manifests as clinical findings (i.e., retinal edema, hemorrhages, hard exudates on fundus examinations, and dye leakage on fluorescein angiography). In physiological retinas, the vascular endothelium functions as an essential barrier, which is supported by neuroglial cells. In diabetic retinas, increased paracellular flux, transcellular transport, and cell death in vascular endothelial cells promote vascular hyperpermeability (5). Among many growth factors and cytokines, VEGF and tumor necrosis factor-a were shown to disrupt the BRB (8,9). In addition to autocrine/paracrine signaling, a classic pathway of vasodilation and inflammation, plasma kallikrein (PKal)-kinin cascade, has recently been revealed to exert potent effects on retinal vascular hyperpermeability (10). Proteomics approaches have discovered a new effect of carbonic anhydrase (CA)-I in the vitreous humor in PDR patients that increases pH and concomitantly activates the kallikrein-kinin pathway (11). In this issue of Diabetes, Kita et al. (12) demonstrate that the PKal-kinin-nitric oxide synthase (NOS) pathway promotes vascular hyperpermeability in DME, independent of the biological effects of VEGF. Biochemical analyses of clinical specimens from DME patients revealed that both PKal and VEGF were increased in the intraocular fluids, although they were not related to each other. The authors conducted interesting translational research to test the hypothesis that the PKal-kinin pathway had VEGF-independent effects on vascular permeability in DME. The vitreous specimens from DME patients containing higher amounts of PKal induced vascular permeability at a longer time point, mediated via bradykinin (BK) B1 or B2 receptors (B1Rs or B2Rs) but not via VEGF signaling. In contrast, the vitreous specimens with high concentrations of VEGF promoted dye leakage at a shorter time point, which did not depend on B1Rs or B2Rs. Further animal experiments confirmed that BK or des-Arg-BK (DABK) increased vascular permeability in diabetic rodents, although antagonists for B1R or B2R did not influence VEGF-induced permeability in vivo. The authors also investigated the functions in NOS as the downstream effectors and showed that endothelial NOS (eNOS) and inducible NOS (iNOS) stimulation by BK and DABK, respectively, promoted retinal vascular permeability. In vivo experiments combined with clinical specimens, to some extent, answered the question raised in the clinic: what are the VEGF-independent molecular mechanisms in DME (12)? As the efficacy of anti-VEGF therapy is significant but limited in DME (7), the authors proposed a novel therapeutic target for vascular permeability in DME (12). Further in vivo pathway analyses using both inhibitors and gene ablation confirmed definitely the detailed molecular mechanisms in this cascade and suggested the candidates for additional molecular targets and possible adverse effects. Intriguingly, several publications reported that B1R and B2R increase the expression of VEGF and its main receptor, VEGFR2, and that B2R has the potential to transactivate VEGFR2 (13,14). BK and VEGF share eNOS activation and Src-mediated VE-cadherin phosphorylation (15,16). Molecules in the PKal-kinin