Silencing of ZnT-1 expression enhances heavy metal influx and toxicity

ZnT-1 reduces intracellular zinc accumulation and confers resistance against cadmium toxicity by a mechanism which is still unresolved. A functional link between the L-type calcium channels (LTCC) and ZnT-1 has been suggested, indicating that ZnT-1 may regulate ion permeation through this pathway. In the present study, immunohistochemical analysis revealed a striking overlap of the expression pattern of LTCC and ZnT-1 in cardiac tissue and brain. Using siRNA to silence ZnT-1 expression, we then assessed the role of ZnT-1 in regulating cation permeation through the L-type Ca²⁺ channels in cells that are vulnerable to heavy metal permeation. Transfection of cortical neurons with ZnT-1 siRNA resulted in about 70% reduction of ZnT-1 expression and increased Ca²⁺ influx via LTCC by approximately fourfold. Moreover, ZnT-1 siRNA transfected neurons showed ∼30% increase in synaptic release, monitored using the FM1-43 dye. An increased cation influx rate, through the LTCC, was also recorded for Zn²⁺ and Cd²⁺ in cells treated with the ZnT-1 siRNA. Furthermore, Cd²⁺-induced neuronal death increased by approximately twofold after transfection with ZnT-1 siRNA. In addition, ZnT-1 siRNA transfection of the ovarian granulosa cell line, POGRS1, resulted in a twofold increase in Cd²⁺ influx rate via the LTCC. Finally, a robust nimodipine-sensitive Cd²⁺ influx was observed using a low extracellular Cd²⁺ concentration (5 μM) in neurons and testicular slice cultures, attesting to the relevance of the LTCC pathway to heavy metal toxicity. Taken together, our results indicate that endogenously-expressed ZnT-1, by modulating LTCC, has a dual role: regulating calcium influx, and attenuating Cd²⁺ and Zn²⁺ permeation and toxicity in neurons and other cell types.