The Role of C-Fos and C-Jun Expression in the Induction of Glutathione S-Transferase 7-7 by Lead Nitrate
Coller, Dale Arold
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Lead toxicity is a major health issue in the United States. It has been linked to metabolic, neurological, growth, and behavioral disorders. To date the biochemical mechanisms of lead toxicity are not well understood. It has been proposed that many effects of lead result from perturbation of signal-transduction processes mediated by a calcium messenger system (Pounds, 1984; Pounds et al., 1991). A potential consequence of altered calcium homeostasis in many tissues is altered expression of intermediate early genes. The placental glutathione Stransferase (GST 7-7) proteins can be induced, both in synthesis and activity, by lead exposure, and are regulated, at least in part, by intermediate early genes. The GST 7-7 gene contains an activation one protein (AP-1) binding site which is activated by dimerized c-fos and c-jun proteins. In this study the effect of lead nitrate on the induction of GST 7-7 through the stimulation of the AP-1 components c-fos and c-jun was examined. Male Sprague-Dawley rats were injected intravenously in with 10 µmol of lead nitrate per kilogram of body weight and were sacrificed at selected time intervals after injection (1, 2, 6, 12, 24, 48, and 72 hours post injection). Cytosolic proteins, nuclear proteins, total RNA and poly (A)+ m RNA were extracted from the liver tissue of these animals and examined using the techniques of SDS-PAGE electrophoresis, western immunoblot analysis, agarose electrophoresis, and P32 labeling of cDNA probes for Northern blot analysis. Through 1,2-dichloro-4- nitrobenzene substrate (CDNB) activity assays and immunoblot analysis of liver cytosol, lead nitrate was found to induce the synthesis of GST' 7-7. Northern Blot analysis of liver poly (A)+ RNA for the first time showed cjun expression to increase 4 fold over the first 6 hours of exposure and cfos expression to increase 10 fold over 48 hours of lead nitrate exposure. The time difference in the increased expression of the proto-oncogenes fos and jun implies that the heterodimer form of the AP-1 transfactor protein is not responsible of the induction of GST 7-7 by exposure to lead.