Université Bordeaux 1 - CNRS, UMR 5805 EPOC, CNRS, Station Marine d'Arcachon, Place du Dr. Peyneau, Arcachon, France.
Aquat Toxicol. 2010 May 5;97(3):260-7. doi: 10.1016/j.aquatox.2009.12.007. Epub 2009 Dec 31.
Metallothioneins (MT) are essential metal binding proteins involved in metal homeostasis and detoxification in living organisms. Numerous studies have focused on MT response to metal exposure and showed an important variability according to species, metal, concentration and time of exposure. In this study, the expression of one isoform of MT gene (Cemt1) and associated MT protein synthesis were determined after 1, 3, 9, 24, 72 and 168h of cadmium (Cd) or mercury (Hg) exposures in gills of the cockle Cerastoderma edule. This experiment, carried out in laboratory conditions, revealed that in Cd-exposed cockles, induction of Cemt1 is time-dependent following a "pulse-scheme" with significant upregulation at 24h and 168h intersected by time point (72h) with significant downregulation. MT protein concentration increases with time in gills of exposed cockles in relation with the progressive accumulation of Cd in soluble fraction. On contrary, Hg exposure does not lead to any induction of Cemt1 mRNA expression or MT protein synthesis compared to control, despite a higher accumulation of this metal in gills of cockles compared to Cd. The localization of Hg (85-90%) is in insoluble fraction, whereas MT was located in the cytoplasm of cells. This gives us a first clue to understand the inability of Hg to activate MT synthesis. However, other biochemical processes probably occur in gills of C. edule since the remaining soluble fraction of Hg exceeds MT sequestration ability. Finally, since one of the first main targets of metal toxicity in cells was the mitochondria, some genes involved in mitochondria metabolism were also analyzed in order to assess potential differences in cellular damages between two metal exposures. Indeed, until T(168), no impact on mitochondrial genes was shown following Hg exposure, despite the complete lack of MT response. This result indicated the presence of other effective cellular ligands which sequester the cytosolic fraction of this metal and consequently inhibit metal reactivity. Such competition mechanisms with other cytosolic ligands more sensitive to Hg were particularly argued in the discussion.
金属硫蛋白（MT）是生物体内金属稳态和解毒过程中必需的金属结合蛋白。大量研究集中在 MT 对金属暴露的反应上，结果表明其根据物种、金属、浓度和暴露时间的不同而存在重要的变异性。在这项研究中，在实验室条件下，研究了贻贝 Cerastoderma edule 鳃中 Cd 或 Hg 暴露 1、3、9、24、72 和 168 小时后，一种 MT 基因（Cemt1）同工型的表达及其相关 MT 蛋白的合成。该实验表明，在 Cd 暴露的贻贝中，Cemt1 的诱导是时间依赖性的，遵循“脉冲方案”，在 24 小时和 168 小时时显著上调，在 72 小时时显著下调。随着暴露贻贝中 Cd 可溶性分数的逐渐积累，MT 蛋白浓度随时间增加。相比之下，与对照相比，Hg 暴露不会导致 Cemt1 mRNA 表达或 MT 蛋白合成的任何诱导，尽管与 Cd 相比，Hg 在贻贝鳃中的积累更高。Hg（85-90%）的定位在不溶性部分，而 MT 位于细胞质的细胞中。这使我们首次了解到 Hg 无法激活 MT 合成的原因。然而，由于剩余的 Hg 可溶性分数超过了 MT 对其的螯合能力，贻贝鳃中可能还发生了其他生化过程。最后，由于金属毒性在细胞中的第一个主要靶标之一是线粒体，因此还分析了一些与线粒体代谢相关的基因，以评估两种金属暴露对细胞损伤的潜在差异。事实上，在 Hg 暴露后，直到 T(168)，线粒体基因没有受到影响，尽管完全没有 MT 反应。这一结果表明，存在其他有效的细胞配体，可以螯合金属的细胞质部分，从而抑制金属的反应性。在讨论中特别强调了与其他对 Hg 更敏感的细胞质配体之间的这种竞争机制。
