Received for publication March 6, 2008.
Revised May 26, 2008.
Accepted for publication June 12, 2008.

Elevated GSH level increases cadmium resistance through downregulation of Sp1-dependent expression of cadmium transporter ZIP8

Abstract

Cadmium is a non-essential toxic metal in mammals. Its toxicity is mainly caused by interactions with cellular proteins that result in protein dysfunction and the disturb normal cellular functions. Glutathione (GSH) has been reported to play a role in cadmium resistance by serving as a cofactor for MRP1/GS-X pump-mediated cadmium elimination. To further investigate the role of GSH in cadmium toxicity, we carried out comparative study using small cell lung cancer derived cell lines, SR3A, and those that were stably transfected with glutamate cysteine ligase catalytic subunit (GCLC), a rate limiting enzyme in GSH biosynthesis. These GCLC-stably transfected cell lines produced higher levels of GSH and were more resistant to cadmium toxicity than the parental cell line was. The rates of cadmium uptake were reduced in these GCLC-transfected cell lines, which were associated with down regulation of the cadmium transporter ZIP8/SLC39A8. Further analyses demonstrated that Sp1 binding site at the proximal promoter region of ZIP8 was sensitive to the GSH level, and that the expression level of transcription factor Sp1 was reduced by increased GSH levels. We also demonstrated that low concentrations of cadmium exposure downregulated ZIP8 expression with concomitant reduction of Sp1 expression. Taken together, these results demonstrate the importance of Sp1 in the regulation of ZIP8 expression. More important, our results reveal a new mechanism by which elevated GSH levels confer cadmium resistance by downregulation of ZIP8 expression through the suppression of Sp1.

Key words: Promoter analysis, Ion transporters (SERCA, Na/K ATPase, CFTR), Regulation of gene expression, Metals and chelators, Toxicant-induced gene express