Proinflammatory Cytokines Disrupt Epithelial Barrier Function by Apoptosis-Independent Mechanisms

Abstract

It is well known that inflammatory conditions of the intestinal mucosa result in compromised barrier function. Inflammation is characterized by an influx into the mucosa of immune cells that influence epithelial function by releasing proinflammatory cytokines such as IFN-γ and TNF-α. Mucosal barrier function is regulated by the epithelial apical junctional complex (AJC) consisting of the tight junction and the adherens junction. Since the AJC regulates barrier function, we analyzed the influence of IFN-γ and TNF-α on its structure/function and determined the contribution of apoptosis to this process using a model intestinal epithelial cell line, T84, and IFN-γ and TNF-α. AJC structure/function was analyzed by confocal microscopy, biochemical analysis, and physiologic measurement of epithelial gate/fence function. Apoptosis was monitored by determining cytokeratin 18 cleavage and caspase-3 activation. IFN-γ induced time-dependent disruptions in epithelial gate function that were potentiated by coincubation with TNF-α. Tight junction fence function was somewhat disrupted. Cytokine treatment was associated with internalization of AJC transmembrane proteins, junction adhesion molecule 1, occludin, and claudin-1/4 with minimal effects on the cytoplasmic plaque protein zonula occludens 1. Detergent solubility profiles of junction adhesion molecule 1 and E-cadherin and their affiliation with “raft-like” membrane microdomains were modified by these cytokines. Inhibition of cytokine-induced apoptosis did not block induced permeability defects; further emphasizing their primary influence on the epithelial AJC structure and barrier function. Our findings for the first time clearly separate the proapoptotic effects of IFN-γ and TNF-α from their abilities to disrupt barrier function.

Neutrophil-Epithelial Interactions

“…Le x engagement also resulted in PMN activation, as evidenced by increased phagocytosis and degranulation. 97 These observations suggest that interactions with endogenous ligands for Le x secreted by bacteria, immune cells, and epithelium, such as Galectin-1, Galectin-3, or even DC-sign, may play potent roles in modulating the immune response at sites of mucosal infection or inflammation.…”

“…106,107 Thus, the expression of JAM-A at epithelial tight junctions serves to inhibit proliferation by keeping activation of Akt in check. 97 with permission from Elsevier, Inc. B: Signaling pathways downstream of cell surface junctional adhesion molecule (JAM)-A that regulate epithelial permeability, proliferation, and migration. CTL, control; AJ, adherens junction; GEF, guanine nucleotide exchange factor; PDZ, PSD-95, discs-large, zo-1; TCF, T cell factor; TJ, tight junction; ZO, zonula occludens protein.…”

LIGHT Signals Directly to Intestinal Epithelia to Cause Barrier Dysfunction via Cytoskeletal and Endocytic Mechanisms

“…While a primary barrier defect may be present in IBD kindreds 12-14 , it is also clear that epithelial barrier defects can be induced by inflammatory cytokines [15][16][17][18][19][20][21][22][23][24] . Recent work has demonstrated that barrier dysfunction induced by inflammatory processes can be due to epithelial damage as well as non-apoptotic regulation of tight junction permeability 19,22,[25][26][27][28] . In vitro and in vivo studies have shown that TNF signals directly to intestinal epithelia to regulate barrier function via myosin light chain kinase (MLCK) activation 20, 29-33 .…”

Inflammatory Bowel Disease Susceptibility Gene C1ORF106 Regulates Intestinal Epithelial Permeability

“…TNF-a inhibitors have been widely used in human IBD, with evidence of mucosal healing in patients that shows clinical benefit (16)(17)(18). We next asked whether C1ORF106 deficiency could exacerbate TNF-a-induced epithelial permeability.…”