Cadmium (Cd2+) is a non-essential heavy metal, which is taken up from the environment into the body through pulmonary and enteral pathways. The S1 segment of the kidney proximal tubule (PT) is a major target of chronic Cd2+ toxicity. Renal dysfunction develops in up to 7% of the general population and in its most severe form displays major features of Fanconi syndrome, such as a defective protein, amino acid, glucose, bicarbonate and phosphate reabsorption. The major pathway for Cd2+ uptake by PT cells (PTCs) in vivo is apical endocytosis of Cd2+ complexed to the high-affinity metal-binding protein metallothionein (MT), which may be receptor-mediated. MT is subsequently degraded in endo-lysosomes, and Cd2+ is liberated for translocation into the cytosolic compartment, possibly using transporters for Fe2+, Zn2+ or Cu2+, such as the divalent metal transporter DMT1. Free Cd2+ ions in the extracellular space are translocated across apical and/or basolateral PTC membranes into the cytosol via transporters, whose identity remains unknown. Cytosolic Cd2+ generates reactive oxygen species (ROS), which deplete endogenous radical scavengers. ROS also damage a variety of transport proteins, including the Na+/K+-ATPase, which are subsequently degraded by the proteasome and endo-lysosomal proteases. Cd2+ causes mitochondrial swelling and release of cytochrome c. If these ROS-mediated stress events are not balanced by repair processes, affected cells undergo apoptosis. But Cd2+ also induces the upregulation of cytoprotective stress and metal-scavenging proteins, such as MT. In addition, Cd2+ upregulates the detoxifying pump multidrug resistance P-glycoprotein, which appears to protect PTCs against Cd2+-induced apoptosis. Thus, Cd2+ interferes with various cellular events ranging from mechanisms of induction of programmed cell death to activation of cell survival genes. A better understanding of the cellular mechanisms involved in Cd2+ nephrotoxicity should provide insights into other heavy metal (e.g. Pb2+, Hg2+) nephropathies and various forms of acquired Fanconi syndrome.

Much of iron and manganese metabolism occurs in mitochondria. Uptake of redox-active iron must be tightly controlled, but little is known about how metal ions enter mitochondria. Recently, we established that the divalent metal transporter 1 (DMT1) is present in the outer mitochondrial membrane (OMM). Therefore we asked if it mediates Fe²⁺ and Mn²⁺ influx. Mitochondria were isolated from HEK293 cells permanently transfected with inducible rat DMT1 isoform 1 A/+IRE (HEK293-rDMT1). Fe²⁺-induced quenching of the dye PhenGreen™SK (PGSK) occurred in two phases, one of which reflected OMM DMT1 with stronger Fe²⁺ uptake after DMT1 overexpression. DMT1-specific quenching showed an apparent affinity of ~1.5 µM for Fe²⁺and was blocked by the DMT1 inhibitor CISMBI. Fe²⁺ influx reflected an imposed proton gradient, a response that was also observed in purified rat kidney cortex (rKC) mitochondria. Non-heme Fe accumulation assayed by ICPOES and stable ⁵⁷Fe isotope incorporation by ICPMS were increased in HEK293-rDMT1 mitochondria. HEK293-rDMT1 mitochondria displayed higher ⁵⁹Fe²⁺ and ⁵⁴Mn²⁺ uptake relative to controls with ⁵⁴Mn²⁺ uptake blocked by the DMT1 inhibitor XEN602. Such transport was defective in rKC mitochondria with the Belgrade (G185R) mutation. Thus, these results support a role for DMT1 in mitochondrial Fe²⁺ and Mn²⁺ acquisition.