Monroe Richard K, Halvorsen Stanley W
Program in Neuroscience, School of Medicine and Biomedical Sciences, University at Buffalo, SUNY, Buffalo, NY 14214-3000, USA.
Neurotoxicology. 2009 Jul;30(4):589-98. doi: 10.1016/j.neuro.2009.03.007. Epub 2009 Apr 1.
Cadmium, mercury and rotenone are environmental pollutants whose neurotoxic mechanisms are not fully understood. We have shown previously that exposure of nerve cells to these agents produces oxidative stress which reversibly blocks growth factor and cytokine-mediated Janus kinase (Jak)/signal transducer and activator of transcription (STAT) signaling. Here we determined a critical role for mitochondrial dysfunction in inhibiting Jak/STAT activity in human BE(2)-C neuroblastoma cells. Exposure of BE(2)-C cells to the heavy metals CdCl(2) and HgCl(2) and to the mitochondrial complex I inhibitor rotenone inhibited interleukin-6, interferon-gamma and ciliary neurotrophic factor-mediated Jak/STAT signaling, reduced Jak1 and Jak2 auto-phosphorylation and induced Jak tyrosine nitration. However, identical exposure of HepG2 hepatoma cells produced no inhibition of these cytokine responses. In contrast, mitochondria in both BE(2)-C and HepG2 cells showed reduced mitochondrial membrane potential and increased superoxide production after exposure to CdCl(2), HgCl(2) and rotenone. Further, in an in vitro Jak auto-phosphorylation assay Jak2 isolated from either BE(2)-C or HepG2 cells was equally inhibited by mitochondria made dysfunctional by treatment with CdCl(2), HgCl(2) and rotenone. Each of these pro-oxidant effects was reversed by the mitochondrial antioxidant alpha-lipoic acid. The actions of cadmium were also blocked by the mitochondrial complex III bypass agent, 2,6-dichloroindophenol. Therefore, in BE(2)-C cells CdCl(2), HgCl(2) and rotenone disrupt mitochondria to increase intracellular ROS, which directly inhibits neuronal Jak tyrosine kinase activity. Non-neuronal cells such as HepG2 cells that are resistant to oxidative stress-mediated inhibition of cytokine signaling possess some as yet unknown mechanism that protects Jak kinases from oxidative insults. Pro-oxidant-induced mitochondrial dysfunction resulting in selective neuronal Jak inhibition provides a potential mechanism for environmental agents to promote neurodegeneration.
镉、汞和鱼藤酮是环境污染物，其神经毒性机制尚未完全明确。我们之前已经表明，将神经细胞暴露于这些物质会产生氧化应激，从而可逆地阻断生长因子和细胞因子介导的Janus激酶（Jak）/信号转导子和转录激活子（STAT）信号通路。在此，我们确定了线粒体功能障碍在抑制人BE(2)-C神经母细胞瘤细胞中Jak/STAT活性方面的关键作用。将BE(2)-C细胞暴露于重金属氯化镉（CdCl₂）、氯化汞（HgCl₂）以及线粒体复合物I抑制剂鱼藤酮，会抑制白细胞介素-6、干扰素-γ和睫状神经营养因子介导的Jak/STAT信号通路，降低Jak1和Jak2的自磷酸化水平，并诱导Jak酪氨酸硝化。然而，相同条件下暴露的HepG2肝癌细胞对这些细胞因子反应没有抑制作用。相反，暴露于CdCl₂、HgCl₂和鱼藤酮后，BE(2)-C细胞和HepG2细胞中的线粒体膜电位均降低，超氧化物生成增加。此外，在体外Jak自磷酸化试验中，从BE(2)-C细胞或HepG2细胞中分离出的Jak2，同样会被用CdCl₂、HgCl₂和鱼藤酮处理而功能失调的线粒体所抑制。这些促氧化作用均可被线粒体抗氧化剂α-硫辛酸逆转。镉的作用也可被线粒体复合物III旁路剂2,6-二氯靛酚阻断。因此，在BE(2)-C细胞中，CdCl₂、HgCl₂和鱼藤酮破坏线粒体以增加细胞内活性氧（ROS），直接抑制神经元Jak酪氨酸激酶活性。像HepG2细胞这样对氧化应激介导的细胞因子信号抑制具有抗性的非神经元细胞，拥有某种尚未明确的机制来保护Jak激酶免受氧化损伤。促氧化剂诱导的线粒体功能障碍导致选择性神经元Jak抑制，为环境因素促进神经退行性变提供了一种潜在机制。
