Characterisation and regulation of iron- and zinc-evoked electrogenic transport in human intestinal epithelial cells
Application of iron or zinc to the apical surface of Caco-2 cells induced an inward short-circuit current (Isc). These I sc responses were concentration-, pH- and temperature-dependent in nature. Fe2+ and Zn2+-induced transport was electrogenic and resulted in intracellular acidification. Transepithelial proton-coupled transport appeared to involve distinct transport mechanisms for each metal. The apical charge needed to generate the Isc responses was carried by the metal ion and protons, whereas basolateral charge carriage was accomplished by protons alone. Transport may occur in any isotonic media, provided that an inwardly-directed proton gradient is supplied across the apical membrane. The magnitude of the Isc responses evoked by Fe2+ and Zn 2+ was dependent upon the development of Caco-2 cells. When DMT1 (IRE) mRNA levels from cells were analysed by real-time PCR, they appeared to follow a similar pattern. These transport processes may be regulated physiologically at the level of the intestine since enterocyte metal status may induce changes in the expression of DMT1 isoforms. Fe2+ and Zn2+ appeared to have different effects on transporter expression and the Isc responses evoked. Pathophysiological and potential physiological regulation of Fe2+ and Zn2+-evoked transport was demonstrated by using 8Br-cGMP to mimic the elevation of intracellular cGMP levels observed during secretory diarrhoea. Exposure of Caco-2 epithelia to 8Br-cGMP inhibited Fe2+ and Zn2+-evoked transport. By using protein kinase inhibitors, it appeared that 8Br-cGMP-depdnent inhibition of transport was mediated by PKG II. Identification of this pathway provides support for its manipulation as a potential strategy in preventing the diarrhoeal-induced iron and zinc malabsorption associated with enterotoxic diarrhoeal diseases.