APOPTOSIS IN THE CORNEA - FURTHER CHARACTERIZATION OF FAS FAS LIGAND SYSTEM/

This study was performed to further characterize expression and functi on of the Fas/Fas ligand system in the cornea, Specifically. these exp eriments examined (1) she effect of genetic inactivation of Fas or Fas ligand genes on keratocyte apoptosis in response to corneal epithelia l wounding, (2) whether cultured human corneal epithelial and endothel ial cells are competent to undergo apoptosis in response to Fas activa tion, (3) expression of membrane bound and soluble Fas and Fas ligand in corneal cells, and (4) the effect of IL-1 on expression of Fas and Fas ligand in corneal fibroblasts. Keratocyte apoptosis in response to corneal epithelial scrape detected by TUNEL assay and transmission el ectron microscopy was significantly decreased, but not eliminated, in Fas ligand -/- mice compared with control +/+ mice, There was also a d ecrease in Fas -/- mice that did not reach statistical significance. T hus, while the Fas/Fas ligand system is likely involved in regulating keratocyte apoptosis in response to epithelial wounding, systems with redundant function probably also modulate this response. Activation of the Fas receptor triggered death with ultrastructural changes charact eristic of apoptosis in corneal epithelial and endothelial cells in cu lture. Since these cell types express both Fas and Fas ligand in vivo, systems must be in place to prevent uncontrolled activation via autoc rine ligand-receptor interaction. Messenger RNAs coding for both membr ane bound and soluble splicing variants of Fas were expressed in each corneal cell type, suggesting that soluble Fas production could be one mechanism to antagonize membrane bound Fas activation. Soluble Fas li gand protein was expressed in wounded ex vivo corneal epithelium, prov iding a mechanism for Fas ligand from epithelium to mediate keratocyte apoptosis. IL-1, however, also stimulated corneal fibroblasts to expr ess Fas ligand mRNA and protein. Therefore, an alternative mode for ep ithelial injury to trigger keratocyte apoptosis may be by release of I L-1, subsequent induction of Fas ligand in keratocytes, and apoptosis mediated by autocrine mechanisms. (C) 1997 Academic Press Limited.