Reactive oxygen and nitrogen species change cellular responses through diverse mechanisms that are now being defined. At low levels, they are signalling molecules, and at high levels, they damage organelles, particularly the mitochondria. Oxidative damage and the associated mitochondrial dysfunction may result in energy depletion, accumulation of cytotoxic mediators and cell death. Understanding the interface between stress adaptation and cell death then is important for understanding redox biology and disease pathogenesis. Recent studies have found that one major sensor of redox signalling at this switch in cellular responses is autophagy. Autophagic activities are mediated by a complex molecular machinery including more than 30 Atg (AuTophaGy-related) proteins and 50 lysosomal hydrolases. Autophagosomes form membrane structures, sequester damaged, oxidized or dysfunctional intracellular components and organelles, and direct them to the lysosomes for degradation. This autophagic process is the sole known mechanism for mitochondrial turnover. It has been speculated that dysfunction of autophagy may result in abnormal mitochondrial function and oxidative or nitrative stress. Emerging investigations have provided new understanding of how autophagy of mitochondria (also known as mitophagy) is controlled, and the impact of autophagic dysfunction on cellular oxidative stress. The present review highlights recent studies on redox signalling in the regulation of autophagy, in the context of the basic mechanisms of mitophagy. Furthermore, we discuss the impact of autophagy on mitochondrial function and accumulation of reactive species. This is particularly relevant to degenerative diseases in which oxidative stress occurs over time, and dysfunction in both the mitochondrial and autophagic pathways play a role.

https://www.science-open.com/document_file/be42339b-b599-4ee8-948b-8cbbb1c79bd4/PubMedCentral/be42339b-b599-4ee8-948b-8cbbb1c79bd4.pdf

Autophagy, mitochondria and oxidative stress: cross-talk and redox signalling

Reactive oxygen species (ROS) are well known for their role in mediating both physiological and pathophysiological signal transduction. Enzymes and subcellular compartments that typically produce ROS are associated with metabolic regulation, and diseases associated with metabolic dysfunction may be influenced by changes in redox balance. In this review, we summarize the current literature surrounding ROS and their role in metabolic and inflammatory regulation, focusing on ROS signal transduction and its relationship to disease progression. In particular, we examine ROS production in compartments such as the cytoplasm, mitochondria, peroxisome, and endoplasmic reticulum and discuss how ROS influence metabolic processes such as proteasome function, autophagy, and general inflammatory signaling. We also summarize and highlight the role of ROS in the regulation metabolic/inflammatory diseases including atherosclerosis, diabetes mellitus, and stroke. In order to develop therapies that target oxidative signaling, it is vital to understand the balance ROS signaling plays in both physiology and pathophysiology, and how manipulation of this balance and the identity of the ROS may influence cellular and tissue homeostasis. An increased understanding of specific sources of ROS production and an appreciation for how ROS influence cellular metabolism may help guide us in the effort to treat cardiovascular diseases.

Reactive Oxygen Species in Metabolic and Inflammatory Signaling.

Carbonylation of proteins is an irreversible oxidative damage, often leading to a loss of protein function, which is considered a widespread indicator of severe oxidative damage and disease-derived protein dysfunction. Whereas moderately carbonylated proteins are degraded by the proteasomal system, heavily carbonylated proteins tend to form high-molecular-weight aggregates that are resistant to degradation and accumulate as damaged or unfolded proteins. Such aggregates of carbonylated proteins can inhibit proteasome activity. A large number of neurodegenerative diseases are directly associated with the accumulation of proteolysis-resistant aggregates of carbonylated proteins in tissues. Identification of specific carbonylated protein(s) functionally impaired and development of selective carbonyl blockers should lead to the definitive assessment of the causative, correlative or consequential role of protein carbonylation in disease onset and/or progression, possibly providing new therapeutic aproaches.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1582-4934.2006.tb00407.x

Protein carbonylation, cellular dysfunction, and disease progression

This article looks back to the antioxidant/free radical field in 1994 and discusses how it has progressed in the past 18 years. In some areas, there has been little change: the role of oxygen radicals and other reactive oxygen species (ROS) in the origin or progression of most human diseases remains uncertain, with cancer and neurodegenerative disease being likely exceptions. Even in diseases in which ROS are involved there has been little progress in developing effective antioxidant treatments. Mega-doses of dietary antioxidants have also generally failed to prevent human disease, in part because they do not decrease oxidative damage in vivo (as revealed by robust biomarkers). However, some strategies that are known to delay disease onset may act, at least in part, by decreasing oxidative damage levels. Nevertheless, far more is known today about endogenous antioxidant defenses and how they are regulated, which has led to a deeper understanding of how some ROS can act as signaling molecules. Increasing endogenous antioxidant levels (e.g., by supplying “pro-oxidants”) may be a better approach to therapeutics and disease prevention than consuming large doses of “dietary antioxidants.”

Free radicals and antioxidants: updating a personal view.

Proteins are major targets for radicals and two-electron oxidants in biological systems due to their abundance and high rate constants for reaction. With highly reactive radicals damage occurs at multiple side-chain and backbone sites. Less reactive species show greater selectivity with regard to the residues targeted and their spatial location. Modification can result in increased side-chain hydrophilicity, side-chain and backbone fragmentation, aggregation via covalent cross-linking or hydrophobic interactions, protein unfolding and altered conformation, altered interactions with biological partners and modified turnover. In the presence of O2, high yields of peroxyl radicals and peroxides (protein peroxidation) are formed; the latter account for up to 70% of the initial oxidant flux. Protein peroxides can oxidize both proteins and other targets. One-electron reduction results in additional radicals and chain reactions with alcohols and carbonyls as major products; the latter are commonly used markers of protein damage. Direct oxidation of cysteine (and less commonly) methionine residues is a major reaction; this is typically faster than with H2O2, and results in altered protein activity and function. Unlike H2O2, which is rapidly removed by protective enzymes, protein peroxides are only slowly removed, and catabolism is a major fate. Although turnover of modified proteins by proteasomal and lysosomal enzymes, and other proteases (e.g. mitochondrial Lon), can be efficient, protein hydroperoxides inhibit these pathways and this may contribute to the accumulation of modified proteins in cells. Available evidence supports an association between protein oxidation and multiple human pathologies, but whether this link is causal remains to be established.

/pdf/protein-oxidation-and-peroxidation-5goudlml51.pdf

Protein oxidation and peroxidation.

Oxidized cytoplasmic and nuclear proteins are normally degraded by the proteasome, but accumulate with age and disease. We demonstrate the importance of various forms of the proteasome during transient (reversible) adaptation (hormesis), to oxidative stress in murine embryonic fibroblasts. Adaptation was achieved by ‘pre-treatment’ with very low concentrations of H2O2, and tested by measuring inducible resistance to a subsequent much higher ‘challenge’ dose of H2O2. Following an initial direct physical activation of pre-existing proteasomes, the 20S proteasome, immunoproteasome and PA28αβ regulator all exhibited substantially increased de novo synthesis during adaptation over 24 h. Cellular capacity to degrade oxidatively damaged proteins increased with 20S proteasome, immunoproteasome and PA28αβ synthesis, and was mostly blocked by the 20S proteasome, immunoproteasome and PA28 siRNA (short interfering RNA) knockdown treatments. Additionally, PA28αβ-knockout mutants achieved only half of the H2O2-induced adaptive increase in proteolytic capacity of wild-type controls. Direct comparison of purified 20S proteasome and immunoproteasome demonstrated that the immunoproteasome can selectively degrade oxidized proteins. Cell proliferation and DNA replication both decreased, and oxidized proteins accumulated, during high H2O2 challenge, but prior H2O2 adaptation was protective. Importantly, siRNA knockdown of the 20S proteasome, immunoproteasome or PA28αβ regulator blocked 50–100% of these adaptive increases in cell division and DNA replication, and immunoproteasome knockdown largely abolished protection against protein oxidation.

Abbreviations: AMC, 7-amino-4-methylcoumarin; BrdU, bromodeoxyuridine; Bz-VGR-AMC, α-N-benzoyl-Val-Gly-Arg-7-amino-4-methylcoumarin; ezrinox, oxidized ezrin; Hbox, oxidized Hb; MEF, murine embryonic fibroblast; PrOxI, hypothesis, protein oxidation and immunoproteasome hypothesis of MHC class I antigen processing; siRNA, short interfering RNA; suc-LLVY-AMC, N-succinyl-Leu-Leu-Val-Tyr-7-amino-4-methylcoumarin; TCA, trichloroacetic acid; Z-LLE-AMC, benzyloxycarbonyl-Leu-Leu-Glu-7-amino-4-methylcoumarin

/pdf/the-immunoproteasome-the-20s-proteasome-and-the-pa28ab-29w6tdwi3b.pdf

The immunoproteasome, the 20S proteasome and the PA28αβ proteasome regulator are oxidative-stress-adaptive proteolytic complexes.

This study examined the hypothesis that postischemic levels of oxidized and/or ubiquitinated proteins may be predictive of functional recovery as they may be indicative of activity of the 20S and/o...

Cheryl Y. Teoh

Papers

The immunoproteasome, the 20S proteasome and the PA28αβ proteasome regulator are oxidative-stress-adaptive proteolytic complexes.

Oxidized and ubiquitinated proteins may predict recovery of postischemic cardiac function: essential role of the proteasome.

Potential roles of protein oxidation and the immunoproteasome in MHC class I antigen presentation: the 'PrOxI' hypothesis.

Oxidative Defense Mechanisms