Heiman A S, Tonner L E
College of Pharmacy and Pharmaceutical Sciences, Florida A & M University, Tallahassee 32307, USA.
Toxicology. 1995 Jun 26;100(1-3):57-68. doi: 10.1016/0300-483x(95)03061-j.
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.
环境污染物铅的具体细胞作用位点尚未完全明确。本研究旨在验证铅暴露会扰乱激素靶组织中糖皮质激素介导的效应这一假说。本研究选用的细胞培养模型是铅对H4-II-C3肝癌细胞中糖皮质激素调节的酪氨酸转氨酶（TAT）比活性的影响。在有无诱导剂地塞米松的情况下，用300 nM - 10 μM醋酸铅处理细胞24或48小时。铅处理24小时和48小时后，分别剂量依赖性地抑制TAT比活性达52%和61%。这些处理时间和醋酸铅浓度并未显著改变细胞总数、[3H]胸苷掺入或台盼蓝排斥率。糖皮质激素受体结合研究表明，未处理细胞的解离常数（Kd）= 8.3 nM，最大结合容量（Bmax）= 290 fmol/mg蛋白质，而在暴露于10 μM醋酸铅48小时的细胞中，Kd = 9.2 nM，Bmax = 262 fmol/mg蛋白质。铅处理并未显著扰乱诱导性糖皮质激素的摄取或初始胞质受体结合事件。为维持诱导的TAT水平，糖皮质激素必须持续存在。撤去类固醇后，铅处理细胞中的酶去诱导作用发生显著改变。地塞米松撤去6小时后，醋酸钠处理细胞中的TAT水平已降至最大值的51%。在醋酸铅处理细胞中，这一水平显著降至最大值的33%。对HTC细胞进行铅处理还显示可改善佛波酯（PMA）对糖皮质激素诱导的TAT活性的放大作用。综上所述，这些结果表明细胞急性暴露于铅可能会抑制激素靶细胞内糖皮质激素介导的酶诱导过程。结果表明，铅可能通过在转录、翻译和/或翻译后水平发挥作用来增加TAT的周转。铅还可能影响糖皮质激素 - TAT信号转导系统中蛋白激酶C（PKC）介导的磷酸化作用。
