Polarized secretion of CXC chemokines by human intestinal epithelial cells in response to Bacteroides fragilis enterotoxin: NF-kappa B plays a major role in the regulation of IL-8 expression

Abstract

Enterotoxigenic B. fragilis, which produces a approximately 20 kD heat-labile toxin (BFT), has been associated with diarrhoeal diseases and mucosal inflammation. To determine if epithelial cells can contribute to BFT-induced inflammation, we assessed the expression of CXC chemokines by BFT-stimulated human intestinal epithelial cells. BFT stimulation increased expression of the neutrophil chemoattractant and activators ENA-78, GRO-alpha, and IL-8. Up-regulated chemokine mRNA expression was paralleled by increased protein levels. Activation of the IL-8 and NF-kappa B transcriptional reporters was inhibited in cells cotransfected with the I kappa B kinase beta and IkB alpha superrepressor plasmids. Whereas lactate dehydrogenase, which was used to monitor cell lysis, was released predominantly from the apical surface, CXC chemokines were predominantly secreted from the basolateral surface of BFT-treated epithelial cells. The basolateral secretion of CXC chemokines from BFT-stimulated colon epithelial cells suggests that these chemokines can contribute to the inflammatory cell infiltrate in the underlying intestinal mucosa.

Fig. 1

Time course of increased CXC chemokine mRNA. Confluent HT-29 monolayers in 24-well plates were incubated with B. fragilis enterotoxin (BFT, 100 ng/ml) for the indicated period. For quantification of CXC chemokine transcripts, total RNA was reverse-transcribed using an oligo(dT) primer and synthetic internal RNA standards, and amplified by PCR. Data are presented as fold-increase in BFT-stimulated ones when compared with the control. The values were expressed as the mean ± SD of five repeated experiments. The ratios of BFT-stimulated/control mRNA levels of IL-8 and GRO-α at time 0 were ∼ 1. Asterisks indicate statistical significance with P < 0·05 in comparison with the control. • IL-8; ▪ GRO-α; ▴ ENA-78.

Fig. 2

CXC chemokine secretion by HT-29 cells stimulated with B. fragilis enterotoxin. Confluent HT-29 monolayers in 24-well plates were incubated with B. fragilis enterotoxin (BFT, 100 ng/ml) for the indicated period and protein levels of each CXC chemokine were determined by ELISA. Data are the mean ± SEM of seven separate experiments. Asterisks indicate statistical significance with P < 0·05 in comparison with the control. ○,bull; IL-8; □,▪ GRO-α; ▵,▴ ENA-78. Open symbols, nonstimulated control; Closed symbols, BFT-stimulated.

Fig. 3

NF-κB activation by HT-29 cells stimulated with B. fragilis enterotoxin. Human intestinal epithelial cell line HT-29 was stimulated with B. fragilis enterotoxin (100 ng/ml). The cells were harvested at each time point up to 2 h after stimulation and NF-κB binding activity was assayed by EMSA. The result is a representative of three repeated experiments.

Fig. 4

Basolateral IL-8 (a) and GRO-α (b) secretion and apical LDH (c) released by polarized Caco-2 cells. Polarized monolayers of Caco-2 cells in Transwell were stimulated with B. fragilis enterotoxin (100 ng/ml) for the indicated period and the supernatants were obtained from upper and lower chambers. □ Apical; ▪ Basolateral. IL-8 and GRO-α secretion were determined by ELISA and LDH activity was determined by enzymatic assay. Data are the mean ± SEM of seven separate experiments.