title: The acute effect of lead acetate on glucocorticoid regulation of tyrosine aminotransferase in hepatoma cells.
abstract: Specific cellular sites of action of the environmental pollutant, lead, have not been completely defined. The present investigations were conducted to test the hypothesis that lead exposure perturbs glucocorticoid-mediated effects in hormonal target tissues. The cell culture model chosen for these investigations was the effects of lead on glucocorticoid-regulated tyrosine aminotransferase (TAT) specific activity in the H4-II-C3 hepatoma cells. Cells were treated with 300 nM-10 microM lead acetate for 24 or 48 h in absence or presence of the inducing agent, dexamethasone. Lead dose-dependently inhibited TAT specific activity up to 52% and 61% following 24 and 48 h lead treatments, respectively. These treatment times and concentrations of lead acetate did not significantly alter total cell numbers, [3H]thymidine incorporation or trypan blue exclusion. Glucocorticoid receptor-binding studies yielded a Kd = 8.3 nM and a Bmax = 290 fmol/mg protein in untreated cells versus a Kd = 9.2 nM and Bmax = 262 fmol/mg protein in cells exposed to 10 microM lead acetate for 48 h. Treatment with lead did not significantly perturb uptake of the inducing glucocorticoids or initial cytosolic receptor-binding events. To sustain induced levels of TAT, glucocorticoid must be continuously present. Following steroid withdrawal, enzyme de-induction was significantly altered in lead-treated cells. At 6 h following dexamethasone withdrawal, TAT levels had decreased to 51% of maximum in sodium acetate-treated cells. This was significantly reduced to 33% of maximum in lead acetate-treated cells. Lead treatment of HTC cells was also shown to ameliorate PMA amplification of dexamethasone-induced TAT activity. Taken together, these results suggest that acute exposure of cells to lead may inhibit processes involved in glucocorticoid-mediated enzyme induction within the hormonal target cell. Results suggest that lead may be acting to increase the turnover of TAT by actions at the transcription, translation and/or posttranslational level. Lead may also be affecting PKC-mediated phosphorylations in the glucocorticoid-TAT signal transduction system.
authors.affiliation: College of Pharmacy and Pharmaceutical Sciences, Florida A & M University, Tallahassee 32307, USA.
authors.forename: A S
authors.initials: AS
authors.lastname: Heiman
authors.forename: L E
authors.initials: LE
authors.lastname: Tonner
mesh_terms: D000818:Animals; D006528:Carcinoma, Hepatocellular / Q000201:enzymology* / Q000473:pathology; D002455:Cell Division / Q000187:drug effects; D003907:Dexamethasone / Q000378:metabolism / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D004347:Drug Interactions; D004785:Environmental Pollutants / Q000633:toxicity; D004790:Enzyme Induction / Q000187:drug effects; D007854:Lead / Q000633:toxicity*; D008113:Liver Neoplasms / Q000201:enzymology* / Q000473:pathology; D009942:Organometallic Compounds / Q000633:toxicity*; D010766:Phosphorylation; D014176:Protein Biosynthesis / Q000187:drug effects / Q000235:genetics; D011493:Protein Kinase C / Q000378:metabolism; D011965:Receptors, Glucocorticoid / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology; D013936:Thymidine / Q000378:metabolism; D014158:Transcription, Genetic / Q000187:drug effects / Q000235:genetics; D014316:Tritium / Q000378:metabolism; D014343:Trypan Blue / Q000378:metabolism; D014407:Tumor Cells, Cultured; D014444:Tyrosine Transaminase / Q000235:genetics / Q000378:metabolism*
chemical_list: D004785:Environmental Pollutants; D009942:Organometallic Compounds; D011965:Receptors, Glucocorticoid; D014316:Tritium; D007854:Lead; D003907:Dexamethasone; D014444:Tyrosine Transaminase; D011493:Protein Kinase C; D014343:Trypan Blue; D013755:Tetradecanoylphorbol Acetate; C008261:lead acetate; D013936:Thymidine