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

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.

In Vitro Effects of Anti-Glaucomatous Eye Drops on Platelet-Activating Factor and its Metabolism

PURPOSE

The purpose of this study is to determine the effect of various commonly used antiglaucoma eye drops on inflammatory mediators such as the platelet activating factor (PAF).

METHODS

Various intraocular pressure (IOP) lowering drops were tested to examine their inhibitory effect on PAF. Multiple eye drops were tested in washed rabbit platelets (WRPs) in order to determine the interaction between these eye drops and the inhibition of PAF in the PAF-induced platelet aggregation model. In addition, we examined the eyedrops' effect on PAF-metabolism, through in vitro analysis on PAF basic metabolic enzymes (PAF-CPT, lyso PAF-AT, and PAF-AH).

RESULTS

Latanoprost (Xalatan) was found to be the most potent in inhibiting PAF, suggesting that it is the most effective in decreasing IOP amongst the eye drops tested. Conversely, dorzolamide hydrochloride-timolol (Cosopt) exhibited the least anti-PAF action.

CONCLUSIONS

This is the first study examine the relationship between PAF activity and glaucoma medication. Potency in PAF inhibition may be related to drop efficacy.

Intravitreally injected platelet activating factor induces retinitis in experimental animals

PURPOSE

Platelet activating factor is a lipid which has been strongly implicated in anterior uveitis. In order to investigate further the role of platelet activating factor in intraocular inflammation, we have characterized the histological changes associated with the intravitreal injection of platelet activating factor, PAF analogs, or lyso-PAF in laboratory rabbits and rats.

METHODS

Initial studies utilized a PAF analog (rac 1-0-octadecyl 2-0-ethyl glycero phosphoryl choline or ethoxy PAF), because this compound is relatively resistant to degradation by hydrolase, the major degradative enzyme for PAF. Doses ranging from 1 ug to 5 mg and time points from 6 hours to 7 days after injection were studied.

RESULTS

In either rats or rabbits, 100 ug of ethoxy PAF consistently induced a marked uveitis with the predominance of inflammation focused in the retina and choroid. Retinitis was also induced in rabbits by either 1 mg PAF injected intravitreally or a similar dose of the PAF precursor/metabolite, lyso PAF. Retinal inflammation was not induced by an inactive lipid, 1,1-0,0-dihexadecyl-rac-glycero-3-phosphocholine, although this compound resulted in mild vitreous inflammation. The histological changes induced by PAF could be readily distinguished from the predominantly anterior inflammation induced by intravitreal injections of substances such an interleukin-1 or endotoxin.

CONCLUSIONS

Recent studies indicating that PAF antagonists inhibit a variety of retinal toxicities and our own observations suggest that PAF could be a major mediator of retinal inflammation.

The role of platelet-activating factor in cell infiltration in endotoxin-induced uveitis in guinea pigs

PURPOSE

We investigated the role of platelet-activating factor (PAF) in cell infiltration in endotoxin-induced uveitis (EIU) in guinea pigs.

METHODS

To elicit EIU, lipopolysaccharide (LPS) was injected into the anterior chamber of the eye. Cell numbers in the aqueous humor after LPS injection were determined by flow cytometry. PAF and prostaglandin E2 (PGE2) production after LPS injection were also examined.

RESULTS

Intracameral injection of LPS induced cell infiltration into the anterior chamber, and platelet-activating factor (PAF) was detected in the aqueous humor. In addition, topical apafant (PAF antagonist) partially inhibited cell infiltration. Intracameral injection of PAF scarcely induced cell infiltration but the reaction with EIU was accelerated by intracameral injection of a small amount of PAF.

CONCLUSIONS

These results suggest that PAF has weak direct activity on cell infiltration in intraocular inflammation but enhances intraocular inflammation.

Involvement of prostaglandin E2 in rabbit corneal injury by anterior segment ischaemia

The involvement of prostaglandins (PGs) in the development of anterior segment ischaemia after occlusion of the bilateral long posterior ciliary arteries was investigated in rabbit eyes. In this experimental ischaemia, the tissue weight and protein content in the peripheral cornea and the protein content in the aqueous humour increased on the first postoperative day. Topically applied cyclooxygenase inhibitor diclofenac (0.1%) reduced corneal inflammation and further suppressed the elevation in the tissue weight and protein content in the peripheral cornea on day 1 after ischaemia, but did not affect the changes in the aqueous humour. Subconjunctivally administered PGE1 and PGE2 induced corneal oedema and increased corneal protein content in diclofenac-treated and ischaemia-induced eyes, but PGD2, PGF2alpha, and the stable PGI2 analogue cicaprost did not evoke any change. In fact, PGE2 content was markedly increased in the aqueous humour on day 1 after ischaemia, and diclofenac suppressed the increase. In addition, CPT-cAMP increased the corneal tissue weight and protein content in organ culture. These observations suggest that PGE2 may play an important role in developing corneal oedema at the initial stage of ischaemic damage, possibly through the cAMP-mediated pathway.

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.

Release of platelet activating factor (PAF) and eicosanoids in UVC-irradiated corneal stromal cells

Ultraviolet (UV) irradiation provokes acute inflammation of the eye, and can be used to model processes that occur in response to damage to the anterior segment. This study characterized ultraviolet-C (UVC, 254 nm) irradiation-induced PAF synthesis, and arachidonic acid (20:4) and eicosanoid release in rabbit corneal stromal cells maintained in vitro. PAF was measured by radioimmunoassay (RIA) after exposing cultured corneal stromal cells to UVC irradiation (20 min, 2, 5, 10 mW/cm2). 14C-20:4-labeled stromal cells were also stimulated with UVC and radiolabeled phospholipids, neutral lipids and eicosanoids were measured. Synthesis of cell-associated and secreted PAF from corneal stromal cells was increased by UV irradiation. UV irradiation (254 nm, 5mW/cm2) enhanced 20:4 release from triacylglycerols, phosphatidylinositol, phosphatidylserine and phosphatidylethanolamine, and increased levels of 20:4-diacylglycerol and unesterified 20:4. The released 20:4 entered both the cyclooxygenase and lipoxygenase pathways after UVC irradiation. The PAF antagonist, BN52021 (10 microM) reduced UVC irradiation-induced stimulation of prostaglandin production, but failed to inhibit UVC-induced 20:4 release and synthesis of lipoxygenase products. Furthermore, exogenous PAF (1 microM) stimulated prostaglandin production, but did not increase the synthesis of lipoxygenase products from radiolabeled 20:4. The effects of PAF on prostaglandin synthesis were inhibited by BN52021. These findings indicate that responses to injury in cultured corneal stromal cells include PAF synthesis, release of 20:4 from glycerolipids, accumulation of diacylglycerol and synthesis of eicosanoids. The data further suggest that during UVC irradiation in vitro, PAF is not a primary or initial mediator of 20:4 release and synthesis of lipoxygenase products, but may mediate UVC-induced prostaglandin synthesis.

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.