Suppression of a high-affinity transport system for manganese in cadmium-resistant metallothionein-null cells

Cadmium is a hazardous heavy metal existing ubiquitously in the environment, but the mechanism of cadmium transport into mammalian cells has been poorly understood. Recently, we have established a cadmium-resistant cell line (Cd-rB5) from immortalized metallothionein-null mouse cells, and found that Cd-rB5 cells exhibited a marked decrease in cadmium uptake. To investigate the mechanism of altered uptake of cadmium in Cd-rB5 cells, incorporation of various metals was determined simultaneously using a multitracer technique. Cd-rB5 cells exhibited a marked decrease in manganese incorporation as well as that of cadmium. However, the reduced uptake of manganese was observed only at low concentrations, suggesting that a high- affinity component of the Mn²⁺ transport system was suppressed in Cd-rB5 cells. Competition experiments and kinetic analyses revealed that low concentrations of Cd²⁺ and Mn²⁺ share the same high-affinity pathway for their entry into cells. The mutual competition of Cd²⁺ and Mn²⁺ uptake was also observed in HeLa, PC12, and Caco-2 cells. The highest uptake of Cd²⁺ and Mn²⁺ by parental cells occurred at neutral pH, suggesting that this pathway is different from a divalent metal transporter 1 that can transport various divalent metals including Cd²⁺ and Mn²⁺ under acidic conditions. These results suggest that a high-affinity Mn²⁺ transport system is used for mammalian cellular cadmium uptake, and that the suppression of this pathway caused a marked decrease in cadmium accumulation in cadmium-resistant metallothionein-null cells.