PLATELET-ACTIVATING-FACTOR ENHANCES UROKINASE-TYPE PLASMINOGEN-ACTIVATOR GENE-EXPRESSION IN CORNEAL EPITHELIUM

Purpose. To determine whether platelet-activating factor (PAF), a lipi d mediator that is accumulated in the cornea after alkali burn, induce s the gene expression of urokinase-type plasminogen activator (uPA) in the corneal epithelium. Possible signaling mechanisms of uPA gene ind uction by PAF also were examined. Methods, Rabbit corneas were culture d with or without PAF. One hour before stimulation, PAF antagonists or other modulators were added to PAF, In some experiments, the corneas were permeabilized to introduce guanosine triphosphate analogs into th e corneal epithelial cells. Corneal epithelia were then harvested for Northern blot analysis, nuclear runoff transcription assay, and zymogr aphy. Results. Platelet-activating factor induced uPA mRNA expression in the corneal epithelium. New protein synthesis was not required for the induction of uPA mRNA. The induction was at the level of transcrip tion as shown by nuclear runoff assays. Additionally, both actinomycin D and alpha-amanitin inhibited the increase in uPA mRNA by PAF, The m essage was translated into protein, which was secreted into the condit ioned medium. An antagonist with high affinity for intracellular PAF b inding sites (BN 50730) inhibited uPA gene expression and cellular sec retion of the protein. The effect of PAF was not mediated by G protein s and was independent of protein kinase C- and cyclic adenosine monoph osphate-dependent signal transduction pathways. Okadaic acid increased the expression of uPA and, at longer times, augmented the effect of P AF, suggesting that a signaling pathway that requires phosphorylation is involved in activated uPA mRNA synthesis. Conclusions. After cornea l injury and inflammation, PAF may be an important initiator of the pr oteolytic cascade, leading to epithelial defects and corneal ulceratio n. Antagonists of PAF could be useful in the prevention of these disea ses.