The N-terminal cysteine is a dual sensor of oxygen and oxidative stress.

Cellular homeostasis requires the sensing of and adaptation to intracellular oxygen (O2) and reactive oxygen species (ROS). The Arg/N-degron pathway targets proteins that bear destabilizing N-terminal residues for degradation by the proteasome or via autophagy. Under normoxic conditions, the N-terminal Cys (Nt-Cys) residues of specific substrates can be oxidized by dioxygenases such as plant cysteine oxidases and cysteamine (2-aminoethanethiol) dioxygenases and arginylated by ATE1 R-transferases to generate Arg-CysO2(H) (R-CO2). Proteins bearing the R-CO2 N-degron are targeted via Lys48 (K48)-linked ubiquitylation by UBR1/UBR2 N-recognins for proteasomal degradation. During acute hypoxia, such proteins are partially stabilized, owing to decreased Nt-Cys oxidation. Here, we show that if hypoxia is prolonged, the Nt-Cys of regulatory proteins can be chemically oxidized by ROS to generate Arg-CysO3(H) (R-CO3), a lysosomal N-degron. The resulting R-CO3 is bound by KCMF1, a N-recognin that induces K63-linked ubiquitylation, followed by K27-linked ubiquitylation by the noncanonical N-recognin UBR4. Autophagic targeting of Cys/N-degron substrates is mediated by the autophagic N-recognin p62/SQTSM-1/Sequestosome-1 through recognition of K27/K63-linked ubiquitin (Ub) chains. This Cys/N-degron-dependent reprogramming in the proteolytic flux is important for cellular homeostasis under both chronic hypoxia and oxidative stress. A small-compound ligand of p62 is cytoprotective under oxidative stress through its ability to accelerate proteolytic flux of K27/K63-ubiquitylated Cys/N-degron substrates. Our results suggest that the Nt-Cys of conditional Cys/N-degron substrates acts as an acceptor of O2 to maintain both O2 and ROS homeostasis and modulates half-lives of substrates through either the proteasome or lysosome by reprogramming of their Ub codes.

Redox potential change by the cystine importer affected on enzymatic antioxidant protection in Deinococcus geothermalis.

Radiation resistant bacteria genus Deinococcus species were well studied on DNA repair and anti-oxidative stress response mechanisms. There are many protection factors as enzymatic and nonenzymatic involved. One of them is intracellular redox potential as like thiol compounds including cysteine acts as primary protectant against oxidation stress. A gene cluster consisting of the genes Dgeo_1986 and Dgeo_1987 of Deinococcus geothermalis was identified as a cystine importer. The expression levels of dgeo_1986 and dgeo_1987 were up-regulated by over 60-fold and 4-fold during the late exponential (L) growth phase, respectively. The double-knockout mutant of dgeo_1986 and dgeo_1987 was reduced in cystine and thiol concentrations and leading to enhanced sensitivity against H2O2 stress. The expression of catalase (Dgeo_2728) as an enzymatic anti-oxidant is more induced in the wild-type strain than the Δdgeo_1986-87 strain at the late growth phase. The expression level of the oxidative stress response regulator OxyR (Dgeo_1888) is dependent on the intracellular redox balance. That is, when the intracellular thiol content was reduced in the wild-type strain during the L phase, OxyR was clearly induced. Interestingly, the expression level of OxyR was higher in the Δdgeo_1986-87 strain than in the wild-type strain upon H2O2 treatment. Although OxyR was induced by H2O2 treatment in Δdgeo_1986-87 strain, where intracellular redox potential of cystine was reduced as a thiol compound due to reduced cystine import, the relative level of expression of catalase was unexpectedly down-regulated. Therefore, the catalase induction system as an enzymatic antioxidant protection should be affected via the cystine importer but not rely on the OxyR controlled manner.

Potentiality of ghrelin as antioxidant and protective agent.

BACKGROUND: Oxidative stress is the result of cellular troubles related to aerobic metabolism. Furthermore, this stress is always associated with biological responses evoked by physical, chemical, environmental, and psychological factors. Several studies have developed many approaches of antioxidant defense to diminish the severity of many diseases. Ghrelin was originally identified from the rat stomach, and it is a potent growth hormone-releasing peptide that has pleiotropic functions. METHODS: A systematic review was conducted within PubMed, ScienceDirect, MEDLINE, and Scopus databases using keywords such as ghrelin, antioxidant, oxidative stress, and systemic oxidative stress sensor. RESULTS: In the last decade, many studies show that ghrelin exhibits protection effects against oxidative stress derived probably from its antioxidant effects. Pieces of evidence demonstrate that systemic oxidative stress increase ghrelin levels in the plasma. The expression of ghrelin and its receptor in ghrelin peripheral tissues and extensively in the central nervous system suggests that this endogenous peptide plays an important role as a systemic oxidative stress sensor. CONCLUSION: The current evidence confirms that ghrelin and its derived peptides (Desacyl-ghrelin, obestatin) act as a protective antioxidant agent. Therefore, stressor modality, duration, and intensity are the parameters of oxidative stress that must be taken into consideration to determine the role of ghrelin, Desacyl-ghrelin, and obestatin in the regulation of cell death pathways.

Oxidative stress induction of DJ-1 protein in reactive astrocytes scavenges free radicals and reduces cell injury.

Astrocytes, one of the predominant types of glial cells, function as both supportive and metabolic cells for the brain. Under cerebral ischemia/reperfusion-induced oxidative conditions, astrocytes accumulate and activate in the ischemic region. DJ-1 has recently been shown to be a sensor of oxidative stress in living cells. However, the function of astrocytic DJ-1 is still unknown. In the present study, to clarify the effect of astrocytic DJ-1 protein under massive oxidative insult, we used a focal ischemic rat model that had been subjected to middle cerebral artery occlusion (MCAO) and reperfusion. We then investigated changes in the distribution of DJ-1 in astrocytes, DJ-1 release from cultured astrocytes, and the effects of recombinant DJ-1 protein on hydrogen peroxide (H(2)O(2))-induced death in normal and DJ-1-knockdown SH-SY5Y cells and on in vitro scavenging of hydroxyl radicals ((*)OH) by electron spin resonance spectrometry. At 24 h after 2-h MCAO and reperfusion, an infarct lesion was markedly observed using magnetic resonance imaging and 2,3,5-triphenyltetrazolium chloride staining. In addition, reactive astrocytes enhanced DJ-1 expression in the penumbral zone of the ischemic core and that DJ-1 protein was extracellularly released from astrocytes by H2O2 in in vitro primary cultures. Although DJ-1-knockdown SH-SY5Y cells were markedly vulnerable to oxidative stress, treatment with glutathione S-transferase-tagged recombinant human DJ-1 protein (GST-DJ-1) significantly inhibited H(2)O(2)-induced cell death. In addition, GST-DJ-1 protein directly scavenged (*)OH. These results suggest that oxidative stress induces the release of astrocytic DJ-1 protein, which may contribute to astrocyte-mediated neuroprotection.