Involvement of Multiple Metal Transporters in Cadmium Transport

抄録

Since cadmium (Cd) is not an essential metal, it is presumed that the entry pathways for other essential metals such as zinc (Zn) and iron (Fe) are utilized for cellular Cd incorporation. However, the precise mechanisms of cellular Cd uptake in mammalian cells still remain unclear. To solve this problem, we have established and characterized two types of Cd-resistant cells from metallothionein (MT)-null (MT-/-) and from MT-expressing (MT+/+) mouse fibroblast cells. The uptake of Cd was extremely suppressed in MT-/- Cd-resistant cells. DNA microarray and subsequent real-time PCR assays revealed that the expression of Zrt, Irt-related protein 8 (ZIP8) was markedly suppressed in MT-/- Cd-resistant cells. The introduction of shRNA of ZIP8 into parental cells resulted in a decrease in Cd accumulation. These data suggest that the down-regulation of ZIP8 plays a pivotal role in the decrease in Cd accumulation and subsequent acquisition of Cd resistance in MT-/- Cd-resistant cells. Furthermore, the uptake of manganese (Mn) was also suppressed in MT-/- Cd-resistant cells, suggesting that ZIP8 is also involved in the uptake of Mn. We also found that Cd resistance in MT+/+ Cd-resistant cells was conferred not only by enhanced expression of MT but also by a decrease in Cd accumulation. The expression of several metal transporters and channels including ZIP8, divalent metal transporter 1 (DMT1), and some voltage-dependent calcium channels were decreased in MT+/+ Cd-resistant cells. Furthermore, MT+/+ Cd-resistant cells exhibited cross-resistance to Mn due to a marked suppression of Mn incorporation. Thus, comparison of expression of metal transporters between MT-/- Cd-resistant cells (IC₅₀ = 30 µM) and MT+/+ Cd-resistant cells (IC₅₀ = 200 µM) suggest that ZIP8 and DMT1 may have different affinities for Cd and Mn, and therefore different roles in the uptake of Cd and Mn.