Effect of thiols on cadmium-induced expression of metallothionein and other oxidant stress genes in rat lung epithelial cells

Abstract

This study examined cadmium-induced alterations in metallothionein-1 (MT), glutathione- S-transferase Ya (GST), and heme oxygenase-1 (HO) gene expression in an adult rat lung epithelial cell line. Elevations in MT mRNA and HO mRNA occurred as early as l h after exposure to a sub-toxic concentration of CdCl 2 (10 μM) whereas GST expression did not increase significantly until 4 h after Cd addition. At t = 8 h, levels of GST, MT, and HO mRNA were elevated 9-fold, 27-fold, and 44-fold, respectively, over basal expression. By 24 h, MT expression was almost back to baseline levels. GST mRNA and HO mRNA were also reduced, compared to 8 h, but to a lesser extent than MT expression. The MT gene was more responsive to low Cd concentrations (5 μM) than the genes for HO or GST whereas HO was induced more than the others at higher Cd doses (10–20 μM). Pro-oxidant conditions play a role in Cd-induced gene expression, as suggested by the rapid decline (15–30 min) in glutathione (GSH), amounting to 25–30% of baseline, that occurred after exposure to 10 μM CdCl 2. This was followed by resynthesis of GSH to a concentration higher than the initial. Depleting GSH by treatment of cells with buthionine sulfoximine (BSO) enhanced Cd-induced expression of MT, GST, and HO whereas thiol supplementation, by treatment with N-acetyl cysteine (NAC), had an attenuating effect. BSO and NAC pretreatment had no effect on basal gene expression or Cd uptake. In summary, this study has shown that: (1) Cd increases MT, GST, and HO gene expression in a time- and dose-dependent fashion; (2) MT gene expression appears to be most sensitive to Cd whereas the HO gene is most inducible at higher Cd concentrations; (3) Cd-induced expression is enhanced by GSH depletion and suppressed by thiol supplementation.