Zrt- and Irt-like protein 8 (ZIP8) and metallothionein aid in zinc protection of cultured vascular endothelial cells against cadmium cytotoxicity

Abstract

Cadmium is a known contributing factor for cardiovascular diseases, such as atherosclerosis and hypertension. Although zinc protects against cadmium cytotoxicity in vascular endothelial cells, the detailed mechanisms, especially the differences in the roles of a metal transporter Zrt- and Irt-like protein 8 (ZIP8) and metallothionein (MT), remain unclear. ZIP8 is involved in the uptake of cadmium, whereas MT is induced by and sequesters it. Zinc protects bovine aortic endothelial cells from cadmium cytotoxicity in a culture system in a concentration-dependent manner. Zinc significantly decreased intracellular cadmium accumulation. Cadmium elevated the expression of ZIP8 mRNA, and zinc significantly suppressed this increase in a concentration-dependent manner. However, the expression of MT was not induced by zinc alone but by cadmium; the induction of MT by cadmium was suppressed by zinc at lower concentrations, but intensified by zinc at higher concentrations. However, even when MT induction was strongly suppressed by siRNA-mediated knockdown, cadmium cytotoxicity was reduced by zinc at both lower and higher concentrations. In the absence of zinc, cadmium cytotoxicity was increased by metal response element-binding transcription factor-1 (MTF-1) siRNA-mediated knockdown of MT-1/2. Consequently, it has been suggested that zinc protects the vascular endothelial cells through a concentration-dependent mechanism. Specifically, the decrease in ZIP8 expression is crucially important for the protective effect of zinc at low concentrations against cadmium cytotoxicity in vascular endothelial cells, whereas both the decrease in ZIP8 and the induction of MT contribute to the protection by zinc at high concentrations.