Inhibitory regulation of glutamate aspartate transporter (GLAST) expression in astrocytes by cadmium-induced calcium influx

After injury to the CNS, the accumulation of extracellular glutamate induces neuronal excitotoxicity, leading to secondary tissue damage. Astrocytes can reduce excess extracellular glutamate primarily through the astrocytic glutamate transporter-1 and the Na⁺-dependent glutamate/aspartate transporter (GLAST). In this study, we used an in vitro model of cadmium-induced cellular stress and found that glutamate uptake activity of astrocytes was suppressed because of cadmium-induced inhibition of GLAST expression. The blockage of cadmium-triggered Ca²⁺ influx by Ca²⁺ chelators elevated GLAST transcription and glutamate uptake activity in astrocytes, suggesting that the suppression of GLAST expression in cadmium-treated astrocytes was Ca²⁺-dependent. This was supported by the findings showing the reduction of GLAST mRNA in astrocytes after treatment with Ca²⁺-ionophore A23187. Cadmium reduced human GLAST promoter activity; however, it increased the binding of Ca²⁺-sensitive activator protein-1 (AP-1) and cAMP response element binding protein (CREB) to their specific elements derived from the human GLAST promoter. These results demonstrate that AP-1 and CREB may be coupled with Ca²⁺-dependent pathway triggered by cadmium to mediate the inhibition of GLAST transcription. Our results suggest that Ca²⁺ influx into astrocytes after CNS injury could cause the down-regulation of GLAST expression, thus reducing the astrocytic glutamate uptake function, which in turn may exacerbate secondary damage after CNS injury.