PAF binding to a single receptor in corneal epithelium plasma membrane

Abstract

Purpose: To study the binding characteristics and the expression of platelet-activating factor receptors (PAF-R) in corneal epithelium to elucidate the site of action of PAF.

Methods: Binding of [3H]PAF was investigated in subcellular fractions of the epithelia of bovine corneas and in membranes from cultured rabbit corneal epithelial cells. Dose-response inhibition curves of [3H]PAF-specific binding were generated using increasing concentrations of several PAF-R antagonists. RNA from rabbit corneal epithelial cells was probed for PAF-R expression by reverse transcription-polymerase chain reaction (RT-PCR) with specifically designed degenerated primers.

Results: Scatchard analysis showed a high-affinity binding site in bovine and rabbit corneal epithelium. The dissociation constant (Kd) and the maximum binding sites (Bmax) in a bovine membrane preparation and similar rabbit fraction were 0.77+/-0.03 nM and 180+/-21 femtomoles/mg protein and 4.3 nM and 1.3 picomoles/mg protein, respectively. Specific PAF-binding sites were found in bovine preparations enriched in plasma membranes with a Kd = 69.6 pM and Bmax = 80 femtomoles/mg protein; no specific binding was found in nuclei or microsomal fractions. RT-PCR of rabbit corneal epithelium generated a single product of the predicted size (478 bp). The deduced amino acid sequence of the purified PCR product was 87% homologous to human PAF-R. The hetrazepines BN 50727 and BN 50730 and the PAF structural analogues CV 3988 and CV 6209 competitively inhibited [3H]PAF binding to corneal epithelium with similar potency. WEB 2086 BS was two orders of magnitude less active in antagonizing PAF binding.

Conclusions: Corneal epithelium contains a single population of receptors localized in the plasma membrane. PAF antagonists exert their actions by blocking this PAF-R. The partial sequence deduced in rabbit corneal PAF-R show a higher homology to the human PAF-R.