Maintaining cellular redox status to allow cell signalling to occur requires modulation of both the controlled production of oxidants and the thiol-reducing networks to allow specific regulatory post-translational modification of protein thiols. The oxidative stress hypothesis captured the concept that overproduction of oxidants can be proteotoxic, but failed to predict the recent finding that hyperactivation of the KEAP1–NRF2 system also leads to proteotoxicity. Furthermore, sustained activation of thiol redox networks by KEAP1–NRF2 induces a reductive stress, by decreasing the lifetime of necessary oxidative post-translational modifications required for normal metabolism or cell signalling. In this context, it is now becoming clear why antioxidants or hyperactivation of antioxidant pathways with electrophilic therapeutics can be deleterious. Furthermore, it suggests that the autophagy–lysosomal pathway is particularly important in protecting the cell against redox-stress-induced proteotoxicity, since it can degrade redox-damaged proteins without causing aberrant changes to the redox network needed for metabolism or signalling. In this context, it is important to understand: (i) how NRF2-mediated redox signalling, or (ii) the autophagy-mediated antioxidant/reductant pathways sense cellular damage in the context of cellular pathogenesis. Recent studies indicate that the modification of protein thiols plays an important role in the regulation of both the KEAP1–NRF2 and autophagy pathways. In the present review, we discuss evidence demonstrating that the KEAP1–NRF2 pathway and autophagy act in concert to combat the deleterious effects of proteotoxicity. These findings are discussed with a special emphasis on their impact on cardiovascular disease and neurodegeneration.

Sirt3 (sirtuin 3) is an NAD-dependent deacetylase localized to mitochondria. Sirt3 expression is increased in mouse muscle and liver by starvation, which could protect against the starvation-dependent increase in oxidative stress and protein damage. Damaged proteins and organelles depend on autophagy for removal and this is critical for cell survival, but the role of Sirt3 is unclear. To examine this, we used Sirt3-KO (knockout) mouse embryonic fibroblast cells, and found that, under basal conditions, Sirt3-KO cells exhibited increased autophagy flux compared with WT (wild-type) cells. In response to nutrient deprivation, both WT and KO cells exhibited increased basal and ATP-linked mitochondrial respiration, indicating an increased energy demand. Both cells exhibited lower levels of phosphorylated mTOR (mammalian target of rapamycin) and higher autophagy flux, with KO cells exhibiting lower maximal mitochondrial respiration and reserve capacity, and higher levels of autophagy than WT cells. KO cells exhibit higher phospho-JNK (c-Jun N-terminal kinase) and phospho-c-Jun than WT cells under starvation conditions. However, inhibition of JNK activity in Sirt3-KO cells did not affect LC3-I (light chain 3-I) and LC3-II levels, indicating that Sirt3-regulated autophagy is independent of the JNK pathway. Caspase 3 activation and cell death are significantly higher in Sirt3-KO cells compared with WT cells in response to nutrient deprivation. Inhibition of autophagy by chloroquine exacerbated cell death in both WT and Sirt3-KO cells, and by 3-methyadenine exacerbated cell death in Sirt3-KO cells. These data suggest that nutrient deprivation-induced autophagy plays a protective role in cell survival, and Sirt3 decreases the requirement for enhanced autophagy and improves cellular bioenergetics.

Mitochondrial production of ROS (reactive oxygen species) is thought to be associated with the cellular damage resulting from chronic exposure to high glucose in long-term diabetic patients. We hypothesized that a mitochondria-targeted antioxidant would prevent kidney damage in the Ins2 +/−AkitaJ mouse model (Akita mice) of Type 1 diabetes. To test this we orally administered a mitochondria-targeted ubiquinone (MitoQ) over a 12-week period and assessed tubular and glomerular function. Fibrosis and pro-fibrotic signalling pathways were determined by immunohistochemical analysis, and mitochondria were isolated from the kidney for functional assessment. MitoQ treatment improved tubular and glomerular function in the Ins2 +/−AkitaJ mice. MitoQ did not have a significant effect on plasma creatinine levels, but decreased urinary albumin levels to the same level as non-diabetic controls. Consistent with previous studies, renal mitochondrial function showed no significant change between any of the diabetic or wild-type groups. Importantly, interstitial fibrosis and glomerular damage were significantly reduced in the treated animals. The pro-fibrotic transcription factors phospho-Smad2/3 and β-catenin showed a nuclear accumulation in the Ins2 +/−AkitaJ mice, which was prevented by MitoQ treatment. These results support the hypothesis that mitochondrially targeted therapies may be beneficial in the treatment of diabetic nephropathy. They also highlight a relatively unexplored aspect of mitochondrial ROS signalling in the control of fibrosis.

The COX (cyclo-oxygenase) pathway generates the reactive lipid electrophile 15d-PGJ 2 (15-deoxy-Δ 12,14 -prostaglandin J 2 ), which forms covalent protein adducts that modulate cell signalling pathways. It has been shown that this regulates important biological responses, including protection against oxidative stress, and supports the proposal that 15d-PGJ 2 has pharmacological potential. Protective pathways activated by 15d-PGJ 2 include those controlling the synthesis of the intracellular antioxidants GSH and the enzyme HO-1 (haem oxygenase-1). The induction of the synthesis of these intracellular antioxidants is, in large part, regulated by covalent modification of Keap1 (Kelch-like erythroid cell-derived protein with cap‘n’collar homology-associated protein 1) by the lipid and the subsequent activation of the EpRE (electrophile-response element). For the first time, we show that the potency of 15d-PGJ 2 as a signalling molecule in endothelial cells is significantly enhanced by the accumulation of the covalent adduct with 15d-PGJ 2 and endogenous Keap1 over the time of exposure to the prostaglandin. The consequence of this finding is that signalling initiated by electrophilic lipids differs from agonists that do not form covalent adducts with proteins because the constant generation of very low concentrations of 15d-PGJ 2 can lead to induction of GSH or HO-1. In the course of these studies we also found that a substantial amount (97–99%) of exogenously added 15d-PGJ 2 is inactivated in the medium and does not enter the cells to initiate cell signalling. In summary, we propose that the accumulation of covalent adduct formation with signalling proteins provides a mechanism through which endogenous intracellular formation of electrophilic lipids from COX can exert an anti-inflammatory effect in vivo .

Nitric oxide (NO) and peroxynitrite both inhibit respiration by brain submitochondrial particles, the former reversibly at cytochrome c oxidase, the latter irreversibly at complexes I–III. Both GSH (IC 50 = 10 μM) and glucose (IC 50 = 8 mM) prevented inhibition of respiration by peroxynitrite (ONOO - ), but neither glucose (100 mM) nor GSH (100 μM) affected that by NO. Thus, unless ONOO - is formed within mitochondria it is unlikely to inhibit respiration in cells directly, because of reactions with cellular thiols and carbohydrates. However, the reversible inhibition of respiration at cytochrome c oxidase by NO is likely to occur (e.g. in the brain during ischaemia) and could be responsible for cytotoxicity.