Prolonged effect of a new platelet-activating factor antagonist on ocular vascular permeability in an endotoxin model of uveitis

PAF-induced inflammatory and immuno-allergic ophthalmic diseases and their mitigation with PAF receptor antagonists: Cell and nuclear effects

Ocular allergies are becoming more prevalent as more airborne pollutants, irritants and microbes pervade our environment. Inflammatory and allergic mediators released by dendritic and mast cells within the conjunctiva cause allergic conjunctivitis (AC), a prevalent ocular surface disorder that affects >40% of the world's human population on a seasonal or perennial basis. Even though histamine is a major culprit, platelet-activating factor (PAF) also contributes to AC, acting either directly or synergistically with histamine and other mediators. PAF receptor-meditated inflammatory reactions, via cell-membrane-bound and nuclear-membrane-bound and nuclear PAF receptors, are also implicated in the etiology of other eye diseases such as uveitis, diabetic retinopathy, corneal and choroidal neovascularization, and age-related macular degeneration which cause serious visual impairment and can lead to blindness. This review highlights the various deleterious elements implicated in the pathological aspects of ocular allergic reactions and inflammation and provides concepts and treatment options to mitigate these eye disorders with a special focus on PAF and PAF receptor antagonists.

Platelet activating factor in the eye: Physiological roles, diseases and future perspectives

Platelet Activating Factor (PAF) is a known phospholipid mediator of inflammation. Since its first description in 1972, it has emerged as a key regulator of vital cellular signaling functions, as proliferation, cell adhesion, and apoptosis. Evidence suggests that interactions between PAF and its receptor (PAFR) play a critical role in nervous system tissues, including the retina. The retina is a very important constituent of the visual system, along with the cornea, sclera, choroid, iris, and ciliary body, that acts synergistically to provide vision and to maintain optical homeostasis. There is evidence that PAF may regulate a wide range of physiological functions in the visual system tissues, such as eye development, inflammation, epithelial wound healing, and synapsis. Due to their multiple functions, PAF and PAFR also have important pathological and clinical implications in ocular disorders such as Choroidal Neovascularization (CNV), Age Macular Degeneration, (AMD), Diabetic Retinopathy (DR), transplant responses, and pharmacological interactions. Studies with PAFR antagonists have shown promising results such as inhibition of neovascularization and chloroquine-induced retinopathies, as well as reducing inflammation and retinal cell death. Due to the importance of PAFR signaling in the visual system and ophthalmology research, this review aims to provide a general overview of current and future perspectives about PAF in eye biology.

Platelet-Activating Factor (PAF) Receptor Antagonism Modulates Inflammatory Signaling in Experimental Uveitis

BACKGROUND

The phospholipid mediator platelet-activating factor (PAF) activates an inflammatory response that includes arachidonic acid release and prostaglandin production in the eye, increasing vascular permeability and inflammation. The purpose of this study is to investigate the action of LAU-0901, a novel PAF receptor antagonist, on experimental uveitis.

METHODS

Uveitis was induced in Lewis rats by lipopolysaccharide treatment. LAU-0901 was then delivered systemically in different concentrations at plus 4 and 16 hours, or vehicle injected as controls. Additional animals were used for histological analyses of untreated, uveitis, and uveitis-plus-LAU-0901 retinas. Conventional histological and immunohistochemical methods were employed. A slit lamp and Spectral Domain-Ocular Coherence Tomography (SD-OCT) retinal imager was used for anterior segment photography and posterior pole OCT. Rats were euthanized 4 hours after the second LAU-0901 injection in this 24-hour model. Aqueous humor was collected and quantified, and also analyzed for tumor necrosis factor alpha (TNF-α).

RESULTS

Uveitic eyes demonstrated hypopyon formation, leukocyte infiltration, and an increase in aqueous protein and TNF-α levels. LAU-0901 treatment resulted in a dose-dependent reduction in inflammation, reflected by reduced total protein levels (up to a 64% reduction). Moreover, hypopyon was prevented, leukocytes were absent in vitreous and aqueous humor, and TNF-α levels were reduced by 91%.

CONCLUSIONS

The PAF receptor antagonist LAU-0901 decreases ocular inflammation in a rat model of anterior uveitis in a dose-dependent manner, suggesting that use of this molecule may provide a means to attenuate inflammation onset and offer a future alternative or adjunctive treatment for ocular inflammation.

Inhibitory effects of the platelet-activating factor receptor antagonists, CV-3988 and Ginkgolide B, on alkali burn-induced corneal neovascularization

PURPOSE

Platelet-activating factor (PAF) has been found in various ocular tissues; the activity of PAF depends on the binding to its specific receptor, PAF-receptor. We investigated the therapeutic effects of PAF-receptor antagonists (CV-3988 and Ginkgolide B) on alkali burn-induced corneal neovascularization (CNV).

METHODS

CNV was induced by applying a 0.2 N sodium hydroxide (3 µl, NaOH) solution directly on mice corneas. CV-3988 (1 mM/10 µl) and Ginkgolide B (1 mM/10 µl) were administered topically on the corneas three times daily for three consecutive days. CNV was evaluated under a slit-lamp microscope. Corneas were processed for histological, immunohistochemical and reverse transcription polymerase chain reaction analysis. Human umbilical vein endothelial cells were used for the migration and tube formation assay.

RESULTS

Application of CV-3988 and Ginkgolide B inhibited CNV caused by alkali burn. CV-3988 and Ginkgolide B attenuated the expression of PAF-receptor mRNA. Alkali injury induced a massively increased intraocular mRNA expression of an angiogenic factor in cornea tissues, whereas these increments were attenuated by the application of CV-3988 and Ginkgolide B.

CONCLUSIONS

CV-3988 and Ginkgolide B reversed opacity and neovascularization in alkali burn-induced corneas. Our findings suggest that CV-3988 and Ginkgolide B may be therapeutically useful in the treatment of CNV and inflammation.

Role of platelet-activating factor in cell death signaling in the cornea: A review

Platelet-activating factor (PAF) is a potent bioactive lipid generated in the cornea after injury whose actions are mediated through specific receptors. Studies from our laboratory have shown that PAF interactions with its receptors activate several transmembrane signals involved in apoptosis. Continuous exposure to PAF during prolonged inflammation increases keratocyte apoptosis and inhibition of epithelial adhesion to the basement membrane. As a consequence, there is a marked delay in wound healing, which is not countered by the action of growth factors. While apoptosis of stroma cells is rapid and potent, epithelial cells as well as myofibroblasts, which appear in the stroma during the repair phase, are resistant to apoptosis. However, PAF accelerates apoptosis of corneal epithelial cells exposed to oxidative stress by stimulating phospholipase A2, producing an early release of cytochrome C from mitochondria and activating caspase-3. In myofibroblasts, PAF has a synergistic action with tumor necrosis factor-alpha (TNF-alpha), increasing apoptosis of the cells to 85%. PAF antagonists block the effects of PAF and could have a therapeutic role in maintaining a healthy and transparent cornea.

Involvement of TNF alpha, IL-1 beta and IL-1 receptor antagonist in LPS-induced rabbit uveitis

The objective of the study was to investigate involvement of TNF alpha, IL-1 beta and IL-1 receptor antagonist (IL-1Ra) in lipopolysaccharide (LPS)-induced uveitis. Intravitreal injection of LPS (100 ng) to rabbits induced a massive leukocyte infiltration and protein leakage into the aqueous humor. Aqueous leukocyte counts and protein levels reached a peak 24 hr after this injection. The peak concentrations of aqueous TNF alpha (230 +/- 37 pg ml-1, at 9 hr) and IL-1 beta (185 +/- 80 pg ml-1, at 18 hr) preceded peak levels of aqueous leukocyte counts and protein levels. In contrast, the levels of aqueous IL-1Ra peaked at 48 hr (12,239 +/- 1964 pg ml-1) and a fairly high concentration of IL-1Ra remained when the inflammatory reactions subsided. Immunohistochemistry and leukocyte-depletion studies showed that infiltrating leukocytes were the major cellular sources of aqueous TNF alpha, IL-1 beta and IL-1Ra. Intravitreal injection of homologous TNF alpha (0.1-1.5 micrograms) or IL-1 beta (0.5-5 ng) reproduced a rapid leukocyte infiltration and protein leakage. Administration of anti-TNF alpha mAb (10 micrograms) suppressed the number of LPS-induced infiltrating neutrophils by 50%, mononuclear cells by 58%, and protein leakage by 42%. Administration of rabbit IL-1Ra (10 micrograms) also suppressed neutrophil influx by 78%, however, neither mononuclear cell influx nor protein leakage was inhibited by rabbit IL-1Ra. Co-administration of the two inhibitors enhanced inhibition of neutrophil infiltration to 88%, and protein leakage to 64%. We conclude that TNF alpha and IL-1 beta are the principal mediators of LPS-induced uveitis. Our observations also suggest that endogenous IL-1Ra may down-regulate inflammatory reactions.

Signal transduction and gene expression in the eye: a contemporary view of the pro-inflammatory, anti-inflammatory and modulatory roles of prostaglandins and other bioactive lipids

Eye tissues respond to physiological and pathophysiological stimuli by the activation of phospholipases and the consequent release from membrane phospholipids of biologically active metabolites. These rapid events have profound effects on long-term ocular physiology. Activation of phospholipase A2 is the first step in the synthesis of two important classes of lipid second messengers, the eicosanoids and platelet-activating factor (PAF). PAF accumulates in the cornea in response to injury. It has been shown to stimulate metalloproteinase gene expression in the corneal epithelium, and is, thus, implicated in the extracellular matrix remodeling that accompanies wound healing and ulceration. PAF antagonists confer protection in animal models of acute and chronic anterior segment inflammation, and block the PAF-enhanced glutamate release from retina. The latter effect suggests a role for PAF in glaucomatous neuronal damage. The eicosanoids, in particular the prostaglandins, have long been implicated in the pathophysiology of ocular inflammation and there is pharmacological evidence for their role in the regulation of intraocular pressure. The induction by PAF of the inducible prostaglandin synthase in neurons and in the corneal epithelium provides a link between the actions of these two lipid second messengers. There may be thresholds of lipid second messenger concentrations which govern their activities as physiological, defensive, or harmful.

Activation of the phospholipase/cyclooxygenase cascade in the rabbit cornea by platelet-activating factor is challenged by PAF receptor antagonists

Platelet-activating factor (PAF) is a potent lipid inflammatory mediator which is generated in the cornea after injury. Its activity is regulated by interaction with specific receptors. The binding of PAF to its receptors initiates biochemical sequences that cluminate in the release of additional lipid mediators. An arachidonoyl-dependent phospholipase A2 is activated to release arachidonic acid from membrane phospholipids, especially phosphatidylcholine and ethanolamine. Arachidonic acid is then predominantly metabolized by the cyclooxygenase pathway to prostaglandins F2 alpha, E2 and D2, whereas the lipoxygenase pathway is not influenced by PAF. The release of arachidonic acid and prostaglandins stimulated by PAF is challenged by the PAF receptor antagonists BN 50727 and BN 50730. PAF acting intracellularly may also induce the synthesis of cyclooxygenase, presumably the 'inducible' isoform PGHS2, which has been implicated in the inflammatory response. Thus, the therapeutic use of PAF receptor angatonists could be potentially beneficial in the management of ocular inflammatory disease.

Ginkgo biloba extract (EGb 761) and a platelet-activating factor antagonist protect the retina in experimental autoimmune uveoretinitis

Oxygen-free radical toxicity is an important factor of tissue necrosis in the eye, especially in the retina. Activation of synthesis and release of platelet-activating factor (PAF) by ocular inflammatory cells and resident cells initiates cascades of mediators and cytokines which contribute to tissue damage in several ocular pathologies. The authors studied the therapeutic effectiveness of Ginkgo biloba extract (EGb 761), a potent free radical scavenger with anti-PAF activity, and of BN 50730, a specific PAF antagonist, on acute experimental autoimmune uveoretinitis induced in rats by S-antigen immunization. These treatments slightly delayed disease onset but had little effect on the severity of uveal inflammation. However, they significantly reduced inflammatory cell infiltration of the retina and the damage of the outer retinal layers. These drugs should become useful adjuvants in the therapy of posterior uveitis and other disorders that might damage the retina.

Intracamerally injected platelet activating factor (PAF) induces marked intraocular inflammatory reactions

An inflammatory response was elicited in the rabbit eye by intracameral injection of platelet activating factor (PAF). PAF induced severe aqueous flare, corneal edema, pupillary constriction and marked biphasic changes in intraocular pressure (IOP) in a dose-dependent manner. All of the responses to PAF were inhibited by the PAF receptor antagonist, BN 52021 (20 mg/kg, i.p.). The cyclooxygenase inhibitor, indomethacin (30 mg/kg, i.p.) caused significant inhibition of the early phase PAF-induced aqueous flare, pupillary constriction and intraocular hypertension, but did not effect PAF-induced corneal edema or intraocular hypotension. NDGA (10 mg/kg, i.p.), a lipoxygenase inhibitor, did not inhibit the inflammatory effects of PAF. PAF-induced chemotactic response was evaluated by tissue chemiluminescence. Intracamerally injected PAF did not significantly increase chemiluminescence in cornea or iris-ciliary body, but intracorneal injection of PAF did cause a chemotactic response in both the conjunctiva and cornea. These data suggest that PAF may be an important mediator of intraocular inflammation and that some PAF-induced effects are prostaglandin dependent, while others may be independent of eicosanoid synthesis and release.

Production of platelet-activating factor in photocoagulated retinas

Platelet-activating factor (PAF), an 1-0-alkyl-2-acetyl-sn-glycero-3-phosphocholine, plays an important role in tissue inflammation and ischemia. Previous results from our laboratory have shown that PAF is synthesized in the cornea after injury and that PAF antagonists reduced inflammation in an experimental model of anterior uveitis. This study was conducted to determine the effect of photocoagulation on PAF levels in the retina. Dutch belted pigmented rabbits underwent panretinal photocoagulation in the right eye with an argon blue-green laser. The left eye of each animal served as the control. Four hours later, the animals were killed. PAF was isolated from retinal extracts using high-performance liquid chromatography (HPLC) and was quantitated by platelet aggregation activity. In each animal the level of PAF in the photocoagulated retina was one-and-one-half to four times higher than in the control retina. The specific PAF antagonist BN52021 completely inhibited PAF activity in each sample. Due to the proinflammatory properties of PAF, its increase after laser application may be implicated in some of the clinical side effects seen after this therapy.