The renal proximal tubule has previously been suggested to contribute to body Fe homeostasis: i. a significant amount of Fe is filtered and subsequently reabsorbed primarily by the renal PT (Wareing et al. 2000); ii. The divalent metal transporter DMT1 is expressed in late endosomes/lysosomes of PT cells (Abouhamed et al. 2006), where its expression correlates inversely with dietary iron intake in rats (Wareing et al. 2003). In the present study, we investigated the expression of ferroportin-1 (FPN-1), an Fe efflux transporter, in rat kidney, and regulation of FNP-1 in a cell line derived from the S1-segment of rat PT under conditions of Fe deprivation and Fe overload. FPN-1 immunoreactivity was found in rat renal cortex and clearly localized to the basolateral membrane of PT cells. Strongest expression was detected in the S2 segment, whereas S1 segments showed moderate and S3 segments much weaker immunoreactivity. FPN-1 was also expressed in the PT cell line at both the mRNA and protein level. Fe depletion by a 36h treatment with 100mM desferrioxamine (DF) did not change overall FPN-1 protein expression in cultured PT cells, as evidenced by immunoblotting. Similarly, Fe overload with 100mM ferric ammonium citrate (FAC; 36h) left the total FPN-1 protein level unaltered, consistent with an unchanged FPN-1 mRNA expression. However, immunofluorescence staining showed an approximately 35% increase in FPN-1 cell surface expression after FAC exposure. This was confirmed using cell surface biotinylation followed by streptavidin precipitation of FPN-1. These results suggest a redistribution of FPN-1 from intracellular compartments to the cell surface during iron overload, possibly allowing for an enhanced Fe disposal from the cells. The renal expression pattern of Fe transport proteins, such as DMT1 and FPN-1, and their regulation by Fe availability, strengthen the notion that the kidney PT is involved in Fe homeostasis.

Supported by DFG & Wellcome Trust