Chronic exposure to cadmium causes preferential accumulation of cadmium in the kidney, leading to nephrotoxicity. In the process of renal cadmium accumulation, the cadmium bound to a low-molecular-weight metal-binding protein, metallothionein, has been considered to play an important role in reabsorption by epithelial cells of proximal tubules in the kidney. However, the role and mechanism of the transport of Cd(2+) ions in proximal tubule cells remain unclear. Zinc transporters such as Zrt, Irt-related protein 8 (ZIP8) and ZIP14, and divalent metal transporter 1 (DMT1) have been reported to have affinities for Cd(2+) and Mn(2+). To examine the roles of these metal transporters in the absorption of luminal Cd(2+) and Mn(2+) into proximal tubule cells, we utilized a cell culture system, in which apical and basolateral transport of metals can be separately examined. The uptake of Cd(2+) and Mn(2+) from the apical side of proximal tubule cells was inhibited by simultaneous addition of Mn(2+) and Cd(2+), respectively. The knockdown of ZIP8, ZIP14 or DMT1 by siRNA transfection significantly reduced the uptake of Cd(2+) and Mn(2+) from the apical membrane. The excretion of Cd(2+) and Mn(2+) was detected predominantly in the apical side of the proximal tubule cells. In situ hybridization of these transporters revealed that ZIP8 and ZIP14 are highly expressed in the proximal tubules of the outer stripe of the outer medulla. These results suggest that ZIP8 and ZIP14 expressed in the S3 segment of proximal tubules play significant roles in the absorption of Cd(2+) and Mn(2+) in the kidney.

The humoral immune response, also called the antibody-mediated immune response, is one of the main adaptive immune systems. The essential micronutrient zinc (Zn) is known to modulate adaptive immune responses, and dysregulated Zn homeostasis leads to immunodeficiency. However, the molecular mechanisms underlying this Zn-mediated modulation are largely unknown. Here, we show that the Zn transporter SLC39A10/ZIP10 plays an important role in B-cell antigen receptor (BCR) signal transduction. Zip10-deficiency in mature B cells attenuated both T-cell-dependent and -independent immune responses in vivo. The Zip10-deficient mature B cells proliferated poorly in response to BCR cross-linking, as a result of dysregulated BCR signaling. The perturbed signaling was found to be triggered by a reduction in CD45R phosphatase activity and consequent hyperactivation of LYN, an essential protein kinase in BCR signaling. Our data suggest that ZIP10 functions as a positive regulator of CD45R to modulate the BCR signal strength, thereby setting a threshold for BCR signaling in humoral immune responses.B lymphocyte | acquired immunity | germinal center | antigen-presenting cell | zinc signaling

Manganese homeostasis involves coordinated regulation of specific proteins involved in manganese influx and efflux. However, the proteins that are involved in detoxification/efflux have not been completely resolved nor has the basis by which they select their metal substrate. Here, we compared six proteins, which were reported to be involved in manganese detoxification/efflux, by evaluating their ability to reduce manganese toxicity in chicken DT40 cells, finding that human ZnT10 (hZnT10) was the most significant contributor. A domain swapping and substitution analysis between hZnT10 and the zincspecific transporter hZnT1 showed that residue Asn 43 , which corresponds to the His residue constituting the potential intramembranous zinc coordination site in other ZnT transporters, is necessary to impart hZnT10's unique manganese mobilization activity; residues Cys 52 and Leu 242 in transmembrane domains II and V play a subtler role in controlling the metal specificity of hZnT10. Interestingly, the His 3 Asn reversion mutant in hZnT1 conferred manganese transport activity and loss of zinc transport activity. These results provide important information about manganese detoxification/efflux mechanisms in vertebrate cells as well as the molecular characterization of hZnT10 as a manganese transporter.

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The mechanisms of cellular cadmium uptake in mammalian cells remain obscure. To solve this problem, we established cadmium-resistant cells (A7 and B5) from metallothionein-null mouse cells, and found that cadmium accumulation was markedly suppressed in these cells. DNA microarray and real-time PCR analyses revealed that expressions of ZIP (Zrt-, Irt-related protein) 8 and ZIP14 were down-regulated in A7 and B5 cells. In particular, both mRNA and protein levels of ZIP8 were markedly suppressed in A7 and B5 cells. Introduction of short hairpin RNA (shRNA) of ZIP8 into parental cells reduced the accumulation of cadmium to about 35% of that of mock-transfected cells, whereas the introduction of shRNA of divalent metal transporter 1 hardly changed cadmium accumulation. Thus, the cadmium resistance in A7 and B5 cells may be conferred primarily by the down-regulation of ZIP8. In mouse tissues, high expression of ZIP8 was noted in the liver, kidney, lung and testis. These data suggest that ZIP8 plays an important role in cellular uptake of cadmium.