Bradykinin enhancement of PGE2 signalling in bovine trabecular meshwork cells

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).

The kallikrein system in retinal damage/protection

Kallikrein is a serine protease involved in the kallikrein-kinnin system. Kallikrein is derived from the blood plasma or tissue, and is correlated with aggravation and improvement in eye diseases, such as, glaucoma, diabetic retinopathy, age-related macular degeneration, and ocular ischemic syndrome. The plasma kallikrein stimulates retinal vascular permeability and intraocular hemorrhage. On the other hand, we had reported that the tissue kallikrein normalizes retinal vasopermeability and inhibited retinal neovascularization and retinal ischemic injury. The protective mechanisms of the tissue-derived kallikrein include the cleavage of vascular endothelial growth factor (VEGF), which suggests that the tissue kallikrein could be potentially-effective against any disease involving the VEGF production.

Characterization of a novel proinflammatory effect mediated by BK and the kinin B₂ receptor in human preadipocytes

Obesity and adipose tissue contribute to local and systemic inflammation. However the role of the inflammatory mediator bradykinin (BK) in this context is not known. We therefore evaluated the effect of BK on adipokines secretion in human preadipocytes during the course of differentiation and characterized the receptors involved. Results obtained from antibody array and ELISA experiments showed that several adipokines are released by human preadipocytes under basal conditions while BK specifically stimulated the production of interleukin(IL)-6 and IL-8. The effect of BK diminished with the progression of differentiation, being almost inactive on adipocytes. In preadipocytes, BK also induced a rapid and transient [Ca²⁺](i) mobilization, a rapid and sustained increase in ERK1/2 activation and enhanced forskolin-stimulated cAMP accumulation. BK was without effect on cell proliferation and viability as assessed by bromodeoxyuridine incorporation, WST-1 conversion, or lactate dehydrogenase leakage and was without effect on adipogenesis as measured by triglyceride accumulation, GPDH activity and leptin release. The B₁ receptor agonist, Lys-[des-Arg⁹]-BK, displayed poor activity or was without effect while overall BK effects were prevented by the selective B₂ receptor antagonist, fasitibant chloride, but not by the B₁ selective antagonist, Lys-[Leu⁸][des-Arg⁹]-BK. Immunoblot analysis and immunofluorescence studies showed that the kinin B₂ receptor was essentially expressed at the beginning of the differentiation program. In conclusion, human preadipocytes expressed kinin B₂ receptors linked to multiple signaling pathways, IL-6 and IL-8 production, and BK proinflammatory response in adipose tissue could be prevented by fasitibant chloride.

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.

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.

Neuroprotective role of bradykinin because of the attenuation of pro-inflammatory cytokine release from activated microglia

Bradykinin (BK) has been reported to be a mediator of brain damage in acute insults. Receptors for BK have been identified on microglia, the pathologic sensors of the brain. Here, we report that BK attenuated lipopolysaccharide (LPS)-induced release of tumor necrosis factor-alpha (TNF-alpha) and interleukin-1beta from microglial cells, thus acting as an anti-inflammatory mediator in the brain. This effect was mimicked by raising intracellular cAMP or stimulating the prostanoid receptors EP2 and EP4, while it was abolished by a cAMP antagonist, a prostanoid receptor antagonist, or by an inhibitor of the inducible cyclooxygenase (cyclooxygenase-2). BK also enhanced formation of prostaglandin E(2) and expression of microsomal prostaglandin E synthase. Expression of BK receptors and EP2/EP4 receptors were also enhanced. Using physiological techniques, we identified functional BK receptors not only in culture, but also in microglia from acute brain slices. BK reduced LPS-induced neuronal death in neuron-microglia co-cultures. This was probably mediated via microglia as it did not affect TNF-alpha-induced neuronal death in pure neuronal cultures. Our data imply that BK has anti-inflammatory and neuroprotective effects in the central nervous system by modulating microglial function.

Kinin modulation of conventional outflow facility in the bovine eye

The aim of this study was to investigate the effects of bradykinin on conventional outflow facility in relation to kinin effects on matrix metalloproteinase (MMP) secretion. Conventional outflow facility was measured in isolated bovine segments perfused at a constant pressure of 10 mmHg. Experiments were also performed in primary cultures of bovine trabecular meshwork cells to assess bradykinin effects on the secretion of MMP-9 assessed by western blot. Administration of bradykinin (10(-7) M) to perfused anterior segments produced a 50% increase in outflow facility above basal levels. The effect was slow to develop, requiring 100 min for a significant increase in facility and 4 h for the peak response to be observed. Pretreatment of anterior segments with the B(2) kinin receptor antagonist, HOE-140 (10(-6) M), or with the nonselective MMP inhibitor, GM6001 (10(-5) M) blocked the response to bradykinin (10(-7) M). Treatment of cultured trabecular meshwork cells with bradykinin (10(-7) M) for 120 min stimulated secretion of MMP-9 into the extracellular media, and this response was inhibited by HOE-140 (10(-6) M). These results demonstrate that bradykinin activates B(2) kinin receptors to increase conventional outflow in the perfused bovine eye and provide evidence that secretion and activation of MMPs within the conventional pathway may mediate the effect.

Anti-inflammatory effects of kinins via microglia in the central nervous system

Kinins are important biologically active peptides that are up-regulated after lesions in both the peripheral and central (CNS) nervous systems. Microglia are immune cells in the CNS and play an important role in the defense of the neuronal parenchyma. In cultured murine microglia, bradykinin (BK) induces mobilization of intracellular Ca2+, microglial migration, and increases the release of nitric oxide and prostaglandin E2. On the other hand, BK attenuates lipopolysaccharide-activated TNF-α and IL-1β release. These results suggest that BK functions as a signal in brain trauma and may have an anti-inflammatory role in the CNS.

Stimulation of cannabinoid (CB1) and prostanoid (EP2) receptors opens BKCa channels and relaxes ocular trabecular meshwork

Prostanoids and cannabinoids have ocular hypotensive and neuroprotective properties. The effect of the prostanoid AH13205 (EP2), the thromboxane-mimetic U46619, the cannabinoid (CB) agonists WIN55212-2 and CP 55,940, endothelin-1 (ET-1) and 8-bromo-cAMP on the membrane currents of trabecular meshwork (TM) cells were measured using the patch-clamp technique and compared to their effects on TM contractility. Previous studies show relaxation of TM to AH 13205 and other substances that elevate cAMP, while U46619 and endothelin-1 contract TM. This study shows that after contraction (100%) with carbachol (10(-6)m), the CB agonist CP 55,940 dose-dependently reduced contractility to 83+/-4% (n=9) (10(-6)m) and 61+/-10%, (n=7) (10(-5)m). In the presence of both the CB1 antagonist AM251 (10(-6)m) and CP 55,940 (10(-5)m), the contractile response to carbachol reached 84+/-3% (n=6) of the original level. In patch-clamp experiments, membrane permeable 8-bromo-cAMP (10(-4)m) had no effect on currents of TM cells. In contrast, AH 13205 and two cannabinoids reversibly enhanced outward current through high-conductance Ca(2+)-activated K(+) channels (BKCa, BK, maxi-K) to the following values (in % of the initial value at 100 mV): AH 13205 (10(-5)m): 200+/-28% (n=6), CP 55,940 (10(-6)m): 196+/-33% (n=7), CP 55,940 (10(-5)m): 484+/-113% (n=7), WIN55212-2 (10(-5)m): 205+/-41% (n=10). Iberiotoxin (10(-7)m) completely blocked these responses. The current response to CP 55,940 (10(-5)m) could be partially blocked by the CB1 antagonist AM251 (10(-6)m). Conversely, the contractile agents in this study either caused a transient reduction in outward current (ET-1(5x10(-8)m)) or had no effect (U46619 (10(-6)m)). We conclude that stimulation of EP2 and CB1 receptors in TM is coupled to the activation of BKCa channels via a non-diffusible second messenger cascade. This effect may contribute to the relaxant activity of EP2 and CB1 agonists in isolated TM strips, modulating ocular outflow.