Nrf2 is activated by disruption of mitochondrial thiol homeostasis but not by enhanced mitochondrial superoxide production

The transcription factor nuclear factor erythroid 2-related factor 2 (Nrf2) regulates the expression of genes involved in antioxidant defenses to modulate fundamental cellular processes such as mitochondrial function and glutathione metabolism. Previous reports proposed that mitochondrial ROS production and disruption of the glutathione pool activate the Nrf2 pathway, suggesting that Nrf2 senses mitochondrial redox signals and/or oxidative damage and signals to the nucleus to respond appropriately. However, until now it has not been possible to disentangle the overlapping effects of mitochondrial superoxide/ hydrogen peroxide production as a redox signal from changes to mitochondrial thiol homeostasis on Nrf2. Recently, we developed mitochondria-targeted reagents that can independently induce mitochondrial superoxide and hydrogen peroxide production (MitoPQ), or selectively disrupt mitochondrial thiol homeostasis (MitoCDNB). Using these reagents, here we have determined how enhanced generation of mitochondrial superoxide and hydrogen peroxide, or disruption of mitochondrial thiol homeostasis affect activation of the Nrf2 system in cells, which was assessed by Nrf2 protein level, nuclear translocation and expression of its target genes. We found that selective disruption of the mitochondrial glutathione pool and inhibition of its thioredoxin system by MitoCDNB led to Nrf2 activation, while using MitoPQ to enhance production of mitochondrial superoxide and hydrogen peroxide alone did not. We further showed that Nrf2 activation by MitoCDNB requires cysteine sensors of Kelch-like ECH-associated protein 1 (Keap1). These findings provide important information on how disruption to mitochondrial redox homeostasis is sensed in the cytoplasm and signaled to the nucleus.