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

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.

The 15-deoxy-δ12,14-prostaglandin J2 inhibits LPS‑stimulated inflammation via enhancement of the platelet‑activating factor acetylhydrolase activity in human retinal pigment epithelial cells

A well-recognized natural ligand of PPARγ, 15-deoxy-δ(12,14)-prostaglandin J(2) (15d-PGJ(2)) possesses immunomodulatory properties. The aim of this study was to elucidate whether 15d-PGJ(2) was able to attenuate lipopolysaccharide (LPS)-induced inflammatory responses in human retinal pigment epithelial (RPE) cells, which are involved in ocular immune responses. In addition, we examined whether the platelet activating factor (PAF) is associated with the anti-inflammatory activity of 15d-PGJ(2). ARPE19 cells treated with varying concentrations of 15d-PGJ(2) and a PAF antagonist (CV3988) were used in this study. The activity of PAF-acetylhydrolase (PAF-AH) was assayed by treatment with 15d-PGJ(2) and CV3988 in the presence of LPS. 15d-PGJ(2) and CV3988 inhibited the LPS-induced mRNA expression and protein production of interleukin-6 (IL-6), monocyte chemoattractant protein-1 (MCP-1), and intercellular adhesion molecule-1 (ICAM-1) in ARPE19 cells. These effects resulting from 15d-PGJ(2) were not abrogated by the PPARγ antagonist, indicating that the actions were PPARγ-independent. Furthermore, 15d-PGJ(2) and CV3988 enhanced the PAF-AH activity. Additionally, 15d-PGJ(2) inhibited the phosphorylation of the extracellular signal-regulated kinase (ERK) and the activation of nuclear transcription factor-κB (NF-κB). These results demonstrated that 15d-PGJ(2) reduced LPS-stimulated inflammatory responses in ARPE19 cells by enhancing the PAH-AH activity. These results suggest that 15d-PGJ(2) may have potent anti-inflammatory activity against ocular inflammation.

Cellular and molecular events in corneal wound healing: significance of lipid signalling

Alterations in the normal healing process after corneal injury can produce undesirable outcomes that range from corneal haze to ulceration and perforation. Lipids play important roles in the complex inflammatory processes that occur after corneal wounding. While some lipid mediators, such as the lipoxygenase derivatives of arachidonic acid, 12-hydroxyeicosatetraenoic acid (12[S]-HETE and 15[S]-HETE), act as second messengers to promote cell proliferation and are possibly involved in the synthesis of other molecules that suppress inflammation, others, such as platelet-activating factor (PAF), exert their actions through specific receptors, play key roles during sustained corneal inflammation (as might occur with chemical burns), and contribute to tissue destruction and neovascularization. PAF is also a strong inducer of selective metalloproteinases (MMPs) that degrade the extracellular matrix. The use of a new PAF antagonist has shown great promise for the treatment of diffuse lamellar keratitis (DLK) and alkali-burned corneas.

Platelet activating factor degradation in tear fluid from guinea pigs with allergic conjunctivitis

The purpose of this study was to investigate the role of platelet-activating factor (PAF) and PAF acetylhydrolase (AH) in conjunctiva. The influence of PAF on conjunctival vascular permeability and the presence of PAF or its metabolites in tears from guinea pigs with allergic conjunctivitis were investigated. We instilled PAF to the eyes of guinea pigs and evaluated vascular permeability. Tear samples were collected from passively sensitized guinea pigs, and the concentration of PAF and its metabolites determined by liquid chromatography-tandem mass spectrometry. Exogenous PAF degradation in tear samples was evaluated with or without diisopropyl fluorophosphate (DFP). Topically applied PAF increased vascular permeability in conjunctiva. In the tear samples from guinea pigs with allergic conjunctivitis, PAF could not be detected. However, 40 +/- 6 ng/ml of lyso-platelet activating factor (lyso-PAF) and 230 +/- 50 ng/ml of 1-alkyl-2-acyl-sn-glycero-3-phosphocholine were detected at 10 min after challenge. Exogenous PAF was rapidly degraded in the tear samples from guinea pigs with allergic conjunctivitis, but not from normal guinea pigs. This PAF degradation was inhibited by DFP. These results suggest that PAF in the tear fluid is quickly hydrolyzed to lyso-PAF by PAF AH, which may be released or activated in allergic conjunctivitis.