Cadmium and mercury suppress cytokine receptor-mediated Jak/STAT activation by inhibiting mitochondrial function and increasing intracellular oxidative stress
Monroe, Richard K.
MetadataShow full item record
The Janus kinase (Jak)/signal transducer and activator of transcription (STAT) pathway is an intercellular signal transduction mechanism that acts in many cell types, including neurons, during development and repair after injury. The loss of Jak/STAT signaling produces significant developmental anomalies and neuropathology. The studies discussed here explore the effects of cadmium- and mercury-induced oxidative stress on neuronal Jak/STAT activation by cytokine receptors. Jak/STAT signaling was selectively inhibited by CdCl 2 and HgCl 2 in cultures of chick retina neurons and neuroblastoma cells, while signaling in the non-neuronal cells HepG2 and chick skeletal myotubes was not affected. We also found that both CdCl 2 and HgCl 2 inhibited ciliary neurotrophic factor (CNTF)-mediated Jak1 and Jak2 tyrosine kinase signaling in human BE(2)-C neuroblastoma cells. CdCl 2 and HgCl 2 also inhibited interferon-γ receptor-mediated STAT phosphorylation, but neither inhibited the fibroblast growth factor receptor tyrosine kinase. Results using dichlorofluorescein indicated CdCl 2 and HgCl 2 doubled cellular oxidative stress, and the antioxidant agents glutathione, N-acetylcysteine and sodium ascorbate protected neurons against CdCl 2 - and HgCl 2 -induced STAT inhibition. We also found that CdCl 2 and HgCl 2 can cause mitochondrial dysfunction, reducing mitochondrial membrane potential (ΔΨ M ) and increasing mitochondrial superoxide free radical production in both BE(2)-C and HepG2 cells. The mitochondrial antioxidant α-lipoic acid reduced superoxide production in isolated mitochondria and restored CNTF-induced STAT phosphorylation in intact BE(2)-C human neuroblastoma cells. Free radical species generated by metal-exposed isolated mitochondria were sufficient to oxidize and inhibit Jak isolated from either BE(2)-C or HepG2 cells, and Jak activity was rescued by treating the mitochondria with α-lipoic acid. These results suggest that CdCl 2 and HgCl 2 initially act by poisoning mitochondria to generate increased ROS and inhibit neuronal Jak/STAT activation. However, Jak activity is protected by some unknown factor in intact HepG2 cells. Neuronal inhibition of Jak kinase by metal-induced oxidative stress is a new mechanism of action which may directly produce neurotoxic symptoms as well as implicate environmental factors in the etiology of neurodegenerative conditions. These results provide a new understanding into ROS-mediated neuropathology, as well as generate future research questions into the nature of cell type specificity to oxidative stress-induced Jak/STAT activation.