Cross-resistance of cadmium-resistant cells to manganese is associated with reduced accumulation of both cadmium and manganese

The mechanism of cellular entry of cadmium remains unclear. We have previously established cadmium-resistant cells from mouse embryonic cells of metallothionein (MT)-null mice, and demonstrated that the down-regulation of a zinc transporter, Zrt/Irt-related protein (ZIP) 8, was responsible for the reduced cadmium incorporation into cells. In the present study, we developed cadmium-resistant cells (A⁺70 and B⁺70) from mouse embryonic cells of MT-expressing wild-type mice. The LC₅₀ values of CdCl₂ for A⁺70 and B⁺70 cells were about 200μM while that of the parental cells was 30μM. We found that the cadmium resistance of these cells was conferred not only by enhanced expression of MT, but also by a decrease in cadmium accumulation. Since the uptake rates of cadmium into A⁺70 and B⁺70 cells were lowered, we determined the expression levels of the metal transporters and channels potentially involved in the cellular uptake of cadmium. We found a down-regulation of multiple transport systems, including ZIP8, divalent metal transporter 1 (DMT1), and α₁ subunits of L-type (Ca_V1.2) and T-type (Ca_V3.1) voltage-dependent calcium channels, in A⁺70 and B⁺70 cells. Furthermore, A⁺70 and B⁺70 cells exhibited cross-resistance to cytotoxicity of MnCl₂, probably due to a marked decrease in manganese uptake in these cells. These results suggest that the suppressed expression of ZIP8 and DMT1, which are known to have affinities for both cadmium and manganese, may be responsible for the reduction in the uptake, and consequently the cytotoxicity, of cadmium and manganese in A⁺70 and B⁺70 cells.