Surface distribution of the EGF receptor during differentiation of the human colon carcinoma cell line HT29-D4

Clostridium difficile toxin B activates the EGF receptor and the ERK/MAP kinase pathway in human colonocytes

BACKGROUND & AIMS

Clostridium difficile toxin B (TxB) mediates acute inflammatory diarrhea characterized by neutrophil infiltration and intestinal mucosal injury. In a xenograft animal model, TxB was shown to induce interleukin (IL)-8 gene expression in human colonic epithelium. However, the precise mechanisms of this TxB response are unknown. The aim of this study was to investigate the TxB-mediated proinflammatory pathway in colonocytes.

METHODS

The effect of TxB on epidermal growth factor receptor (EGFR), extracellular signal-regulated kinase (ERK) 1/2 signaling pathway and IL-8 gene expression was assessed in nontransformed human colonic epithelial NCM460 cells. TxB regulation of EGFR-ERK1/2 signaling pathways was determined using immunoblot analysis, confocal microscopy, and enzyme-linked immunosorbent assay, whereas IL-8 gene expression was measured by luciferase promoter assay.

RESULTS

TxB activates EGFR and ERK1/2 phosphorylation with subsequent release of IL-8 from human colonocytes. Pretreatment with either the EGFR tyrosine kinase inhibitor, AG1478, or an EGFR-neutralizing antibody blocked both TxB-induced EGFR and ERK activation. By using neutralizing antibodies against known ligands of EGFR, we found that the activation of EGFR and ERK1/2 phosphorylation was mediated by transforming growth factor-alpha (TGF-alpha). Inhibition of matrix metalloproteinase (MMP) decreased TGF-alpha secretion and TxB-induced EGFR and ERK activation. Inhibition of MMP, EGFR, and ERK activation significantly decreased TxB-induced IL-8 expression.

CONCLUSIONS

TxB signals acute proinflammatory responses in colonocytes by transactivation of the EGFR and activation of the ERK/MAP kinase pathway.

Membrane receptor location defines receptor interaction with signaling proteins in a polarized epithelium

Signal transduction from receptors is mediated by the interaction of activated receptors with proximate downstream signaling proteins. In polarized epithelial cells, the membrane is divided into subdomains: the apical and basolateral membranes. Membrane receptors may be present in one or both subdomains. Using a combination of immunoprecipitation and Western blot analyses, we tested the hypothesis that a tyrosine kinase growth factor receptor, epidermal growth factor receptor (EGFR), interacts with distinct signaling proteins when present at the apical vs. basolateral membrane of a polarized renal epithelial cell. We report here that tyrosine phosphorylation of phospholipase C-gamma (PLC-gamma) was induced only when basolateral EGFR was activated. In contrast, tyrosine phosphorylation of several other signaling proteins was increased by activation of receptor at either surface. All signaling proteins were distributed diffusely throughout the cytoplasm; however, PLC-gamma protein also displayed a concentration at lateral cell borders. These results demonstrate that in polarized epithelial cells the array of signaling pathways initiated by activation of a membrane receptor is defined, at least in part, by the membrane location of the receptor.

Convergent effects of growth factors, hormones, and fibronectin are necessary for the enterocyte differentiation of a colon adenocarcinoma cell line (HT29-D4)

The aim of this work was to show in serum-free medium a convergent effect of physiological factors and extracellular matrix proteins on the differentiation process of enterocytes by taking as a model the HT29-D4 clone that has the feature of differentiating when subcultured in fetal bovine serum glucose-free medium. We show that triiodothyronine (T3) as well as insulin promotes limited cell growth and differentiation, whereas fibronectin or bovine serum albumin (BSA) induces cell growth and a low level of differentiation. However, insulin, T3, fibronectin, and BSA together with epidermal growth factor and transferrin promoted satisfactory growth and enterocyte morphology with epithelial electrophysiological properties in HT29-D4 cells. With these factors adequate protein targeting was achieved since cells apically expressed the carcinoembryonic antigen, and basolaterally transferrin and insulin receptors, beta 1 and alpha v beta 6 integrins, talin, vinculin, and focal adhesion kinase (FAK). Talin, vinculin, FAK, and alpha v beta 6 integrin, the fibronectin receptor, were clustered in focal contacts, which agrees with a possible role of fibronectin in final cell growth, the latter process mediating the final phase of differentiation. This level of differentiation can be maintained for a long time. Thus HT29-D4 cells appear to be a suitable model to study the implication of integrins in the differentiation process of human enterocytes.