A Symposium Session, Molecular Mechanisms Involved in Neuro/Glial Toxicity: From Oxidative Stress to Redox Signal Transduction, was held on March 26, 2014 in conjunction with the 53rd Annual Meeting of the Society of Toxicology in Phoenix, Arizona. The summary prepared by Dr. Leeds is focused on one of the speakers.To read the full abstract of this session, please visit the SOT 2014 Annual Meeting Mobile Event App.
Rodrigo Franco and Michelle Block chaired the session. Dr. Franco briefly introduced the concept of convergence of toxicant treatment leading to oxidative stress. We now refer to this as “redox signaling.” It is technically the imbalance between ROS and defenses against ROS that lead to oxidative damage.This may be more appropriately defined as disruption in redox signaling.
Manisha Patel gave a well-organized presentation on the relationship between redox cycling and Parkinson’s Disease (PD), which is a disease in which the role of ROS has been established through sufficient evidence from both animal and human studies showing oxidative and nitrosive damage in PD. Environmental damage combines with genetic factors and other neurotransmitter (particularly dopaminergic) dysfunction to cause damage. MPTP is a protoxin inhibitor of Complex I in mitochondria.This chemical toxin is useful for in vivo and in vitro modeling. Exposure to a similar chemical, paraquat, increases the risk of developing PD. Animal studies have shown increased synuclein aggregation, lipid peroxidation, and protein nitration after exposure to paraquat. In addition, Superoxide dismutase (SOD), an oxygen radical scavenger, can attenuate the toxicity, indicating a redox mechanism of toxicity.
Dr. Patel looked at production of ROS in mitochondria after PQ exposure in dopaminergic cells (N27) and found that PQ increased H2O2 production that could be blocked by catalase or inhibition of Complex III. Polarography methods were used to show detoxification of H2O2 occurred by removal of the H2O2 by intact mitochondria, which was not dependent on the electron transport chain (ETC), and specific for brain but not liver mitochondria. Enzymatic system that removes H2O2 and found that at low H2O2 concentration the thioredoxin system is responsible for H2O2 removal. Catalase was clearly not responsible.The greatest inhibition was observed with Auranofin (inhibitor of thioredoxin). Auranofin exacerbated ROS generation and prevented removal.
Experiments with subtoxic PQ levels increased production of H2O2 only with Auranofin but not with PQ alone, showing exacerbation of toxicity when thioredoxin was inhibited. Short hairpin RNA knockdown experiments showed potentiation of PQ toxicity while overexpression of thioredoxin resulted in reduced toxicity after PQ exposure, confirming the role of thioredoxin in detoxification of PQ.
The last series of experiments Dr. Patel discussed were evaluating the role of mitochondrial aconitase in neurotoxicity. Normally aconitase catalyzes the transformation of citrate to isocitrate. However, following oxidative inactivation higher levels of superoxide anion are formed and contribute to toxicity. Dr. Patel detailed elegant experiments showing that this toxicity is likely due to Fenton reaction toxicity that results when aconitase loses one of its four irons in the ferrous form, which participates in the Fenton reaction, resulting in the release of hydroxide radicals.