BISPHOSPHONATES INCREASE TIGHT JUNCTION PERMEABILITY IN THE HUMAN INTESTINAL EPITHELIAL (CACO-2) MODEL

The human Caco-2 monolayer model was used to elucidate the potential role of bisphosphonates in the regulation of the junctional complex opening. Experiments were performed in parallel with EGTA, a potent free Ca2+ chelator, which was already demonstrated to increase intestinal epithelium permeability both in vitro and in vivo. Following treatment of Caco-2 cells with increasing non-toxic concentrations of tiludronate, a new bisphosphonate, different phenomena could be observed such as (i) free Ca2+ complex formation at tiludronate concentrations above 0.3 mM and a Ca2+ IC50 around 10-20 mM, (ii) increase in monolayer permeability, i.e., decrease in the transmembrane epithelial electrical resistance, at tiludronate concentrations above 10 mM and a TEER IC50 around 40 mM, associated with (iii) an opening of the junctional complex (desmosomes, intermediate and tight junctions) as demonstrated by scanning electron microscopy of the cell monolayer and electron microscopy of the apical pole of cells. (iv) The direct consequence of these observations was an increase in monolayer permeability at the level of the paracellular route. Hence, the paracellular transport not only of the PEG(400) probe was increased following treatment of Caco-2 monolayers with tiludronate concentrations above 20 mM but also that of tiludronate itself at concentrations above 15 mM. These results are in agreement with a previous study (Boulenc et al., Biochem. Pharmacol., 46 (1993) 1591-1600) which showed that tiludronate was specifically transported across Caco-2 cells via the paracellular route. Therefore, by its specific effect on free Ca2+ concentration, tiludronate could regulate the junctional complex opening of the intestinal epithelium monolayer and enhance its own transport across the intestinal wall.