Received for publication March 16, 2006.
Revised April 24, 2006.
Accepted for publication April 25, 2006.

ZIP8, member of the solute-carrier-39 (SLC39) metal-transporter family: characterization of transporter properties

Abstract

Cadmium (Cd) is a dangerous metal distributed widely in the environment. Recently, this laboratory identified the ZIP8 transporter protein, encoded by the mouse Slc39a8 gene, to be responsible for genetic differences in response to Cd damage of the testis. Stable retroviral infection of the ZIP8 cDNA, in mouse fetal fibroblast (MFF) cultures (rvZIP8 cells), leads to as much as a 10-fold increase in the rate of intracellular Cd influx and accumulation. In the present study we show that Cd uptake operates maximally at pH 7.5 and temperature of 37 °C, and inhibited by cyanide. Of more than a dozen cations tested, manganese(II) (Mn) is the best competitive cation for Cd uptake. The K_m for Cd²⁺ uptake is 0.62 µM and the K_m for Mn²⁺ uptake is 2.2 µM; thus, Mn is likely the physiological substrate for ZIP8. Cd uptake is independent of sodium, potassium or chloride ions, but strongly dependent on the presence of bicarbonate. By Western blot analysis of rvZIP8 cells, we show that ZIP8 protein is glycosylated. Using Z-stack confocal microscopy in Madin-Darby canine kidney (MDCK) polarized epithelial cells, we found that ZIP8 is localized on the apical side--implicating an important role for Mn or Cd uptake and disposition. It is possible that ZIP8 is a Mn²⁺/HCO₃- symporter, that a HCO₃- gradient across the plasma membrane acts as the driving force for Mn uptake, and that Cd is a rogue hitchhiker displacing Mn to cause Cd-associated disease.

Key words: Ion transporters (SERCA, Na/K ATPase, CFTR), Genetics, Metals and chelators, Oxidative stress/antioxidants, Oxidative stress

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