Induction of 78kD glucose-regulated protein (GRP78) expression and redox-regulated transcription factor activity by lead and mercury in C6 rat glioma cells

Lead (Pb) and mercury (Hg) are widespread environmental contaminants that induce prominent neural toxicity. Although the brain is not the major Pb and Hg depot in the body, these metals preferentially accumulate in astroglia to exert toxic effects. In this study, we examined the effects of Pb acetate and HgCI₂ on the expression of GRP78, a molecular chaperone in the endoplasmic reticulum (ER) that may provide cytoprotection in response to cellular stresses in the C6 rat glioma cell line. We also evaluated the DNA binding activities of several redox-regulated transcription factors in metal-treated cells. Our results showed that mRNA levels of GRP78 were up-regulated by Pb and Hg at 0.1 and 1 μM, but down-regulated at higher concentrations (10 μM). GRP78 protein levels increased in a concentration- and time-dependent manner in Pb and/or Hg-treated cells. Pb increased protein binding to the GST-Ya antioxidant/electrophile response element (ARE/EpRE) and to the NF-κB consensus binding sequence of the cytomegalovirus 2 (CMV2) promoter, but decreased protein binding to the Ha-ras ARE/EpRE or to the c-fos 12-O-tetradecanoyl-phorbol-13-acetate (TPA) response element (TRE). In contrast, Hg activated DNA binding by all redox-regulated transcription factors. These studies shed some light on the molecular mechanisms of Pb and Hg toxicity in C6 rat glioma cells and suggest that GRP78 and oxidative stress may participate in the neurotoxic response to these metals.