hPepT1-mediated epithelial transport of bacteria-derived chemotactic peptides enhances neutrophil-epithelial interactions.

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Abstract
Intestinal epithelial cells express hPepT1, an apical transporter responsible for the uptake of a broad array of small peptides. As these could conceivably include n-formyl peptides, we examined whether hPepT1 could transport the model n-formylated peptide fMLP and, if so, whether such cellular uptake of fMLP influenced neutrophil-epithelial interactions. fMLP uptake into oocytes was enhanced by hPepT1 expression. In addition, fMLP competitively inhibited uptake of a known hPepT1 substrate (glycylsarcosine) in hPepT1 expressing oocytes. hPepT1 peptide uptake was further examined in a polarized human intestinal epithelial cell line (Caco2-BBE) known to express this transporter. Epithelial monolayers internalized apical fMLP in a fashion that was competitively inhibited by other hPepT1 recognized solutes, but not by related solutes that were not transported by hPepT1. Fluorescence analyses of intracellular pH revealed that fMLP uptake was accompanied by cytosolic acidification, consistent with the known function of hPepT1 as a peptide H+ cotransporter. Lumenal fMLP resulted in directed movement of neutrophils across epithelial monolayers. Solutes that inhibit hPepT1-mediated fMLP transport decreased neutrophil transmigration by approximately 50%. Conversely, conditions that enhanced the rate of hPepT1-mediated fMLP uptake (cytosolic acidification) enhanced neutrophil-transepithelial migration by approximately 70%. We conclude that hPepT1 transports fMLP and uptake of these peptide influences neutrophil-epithelial interactions. These data (a) emphasize the importance of hPepT1 in mediating intestinal inflammation, (b) raise the possibility that modulating hPepT1 activity could influence states of intestinal inflammation, and (c) provide the first evidence of a link between active transepithelial transport and neutrophil-epithelial interactions.

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