Divalent metal transporter 1 in the kidney proximal tubule is expressed in late endosomal/lysosomal membranes: implications for renal handling of protein‐metal complexes

The H + coupled divalent metal transporter 1 (DMT1) plays a key role in iron homeostasis. It has a widespread pattern of expression including tissues associated with iron acquisition and storage. Interestingly, it is also highly expressed in kidney, yet its function there is unknown. In this study we determined the cellular location of DMT1 in proximal tubule (PT) cells as a first step to identifying the role of this protein in the kidney. We performed RT‐PCR and immunostaining experiments using rat kidney and the S1 PT derived WKPT‐0293 Cl.2 cell line. RT‐PCR revealed that mRNAs encoding all four DMT1 splice variants were present in rat kidney cortex or cultured PT cells. Immunostaining showed that DMT1 protein was expressed intracellularly and was not present in the plasma membrane. Expression of DMT1 partially co‐localized with the late endosomal and lysosomal proteins LAMP1 and cathepsin‐L. Using immunogold labeling, DMT1 was shown to be expressed in the membranes of late endosomes/lysosomes. Uptake of Alexa Fluor546‐transferrin was only observed following application to the apical membrane of cultured PT cells and co‐localized with DMT1. In conclusion, renal PT cells express DMT1 in the membranes of organelles, including late endosomes/lysosomes, associated with processing of apically sequestered transferrin. These findings have implications for renal iron handling and for the handling of nephrotoxic metals that are also DMT1 ligands including Cd 2+ .