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 poor ly 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 techniq ue. Cd-rB5 cells exhibited a marked decrease in manganese incorporation as well as that of cadmium. However, the reduced uptake of manganese was obser ved only at low concentrations, suggesting that a high-affinity component o f the Mn2+ transport system was suppressed in Cd-rB5 cells. Competition exp eriments and kinetic analyses revealed that low concentrations of Cd2+ and Mn2+ share the same high-affinity pathway for their entry into cells. The m utual 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 occ urred at neutral pH, suggesting that this pathway is different from a dival ent metal transporter 1 that can transport various divalent metals includin g Cd2+ and Mn2+ under acidic conditions. These results suggest that a high- affinity Mn2+ transport system is used for mammalian cellular cadmium uptak e, and that the suppression of this pathway caused a marked decrease in cad mium accumulation in cadmium-resistant metallothionein-null cells.