Trefoil Factor TFF1-Induced Protection of Conjunctival Cells from Apoptosis at Premitochondrial and Postmitochondrial Levels

Abstract

Goblet cells of the conjunctival epithelium synthesize and secrete TFF1 (Trefoil factor 1), a small protease-resistant peptide that, together with mucins, is responsible for the rheologic properties of the tear film. This study aimed to determine whether TFF1, whose synthesis increases in inflammatory conditions such as pterygium, could protect conjunctival cells from apoptosis. Chang conjunctival cells, either wild-type or expressing TFF1 through stable transfection, were exposed to benzalkonium chloride (BAK) and ultraviolet (UV) irradiation to trigger apoptosis. The authors used cell fractionation to detect lipid raft-associated proteins, coimmunoprecipitation to explore the formation of a death-inducing signaling complex (DISC), and a combination of immunofluorescence, immunoblotting, flow cytometry, siRNA-mediated decrease in gene expression, and electrophoretic mobility shift assay to explore the mechanisms of TFF1-protective effects. TFF1 protects Chang conjunctival cells from apoptosis induced by UV irradiation and BAK at two levels. First, TFF1 prevents caspase-8 activation at the level of the DISC that involves Fas receptor in plasma membrane rafts, which in turn decreases the mitochondrial release of cytochrome c. Second, TFF1 interferes with caspase-9 and caspase-3 activation through an NF-kappaB-induced increase in the expression of XIAP (X-linked inhibitor of apoptosis protein). TFF1 upregulation on inflammatory conditions may be a protective mechanism that limits conjunctival cell loss by inhibiting apoptosis upstream and downstream of the mitochondrial events. These observations suggest a potential interest of TFF1 or related peptides to prevent cell death in ocular surface disorders.