Oxidative stress: an overview of past research and future insights

Abstract

‘Oxidative stress' (OS) refers to the deviations in redox biology inside biological systems. This term has been used to illustrate the imbalance between oxidants and antioxidants, which has been a subject of extensive research. The obtained findings suggest that many diseases are linked to OS, including diabetes mellitusinflammation, cancers, autoimmune disorders and neurodegenerative diseases. More recently, significant advances in biochemistry and molecular biology have enabled researchers to find master switches of various OS mechanisms and associate OS development to many biological paradigms. In this review, a brief overview of OS research is provided to offer insight into its future directions. Most importantly, the need for a greater understanding of the OS molecular mechanisms and its vital role in the progression of many disorders is emphasized. Likewise, more work is needed on the utility of reactive oxygen species markers as a diagnostic tool.

KEYWORDS:

• Molecular switches
• oxidative stress
• proteomics
• redox biology

1. Introduction

The concept of oxidative stress (OS) first emerged in 1985 in reference to an imbalance between the oxidants which attack the cell and the antioxidants responsible for defending it, leading to the disruption in redox signaling and control, and thus causing molecular damage (Sies 2015). This hypothesis prompted extensive research on the relationship between oxidants and antioxidants, their sources and metabolic aspects, as well as their implications for radiology, radiation biology, chemistry, biochemistry and medicine (Zimmerman and Case 2019; Gomez-Contreras et al. 2021; Lorenzen et al. 2021). The obtained findings indicate that alterations in the oxidation−reduction reactions inside the cells will generate reactive oxygen species (ROS) which are synthesized through aerobic metabolic processes involving oxygen arising from the action of many oxidases such as lipoxygenases (LPO), xanthine oxidase (XO), nicotinamide adenine dinucleotide phosphate (NADPH) oxidases, cyclooxygenases (COX), uncoupled nitric oxide synthases (NOS), and cytochrome P450 enzymes (Mueller et al. 2005). In healthy individuals, small amounts of ROS, such as hydroxyl radical (OH), superoxide (O₂⁻), hydrogen peroxide (H₂O₂), and peroxynitrite (ONOO⁻), are overcome by the action of antioxidants. However, under pathological conditions, ROS production exceeds the generation and power of antioxidants, leading to OS (Li et al. 2013). Figure 1.

Figure 1. Different resources of reactive oxygen and nitrogen species.

ROS production will lead to multiple cellular dysfunctions (Vaziri and Rodriguez-Iturbe 2006; Pisoschi et al. 2021). However, it is attractive for many researchers; the OS research field has many merits and pitfalls (Pompella et al. 2014). Moreover, the concept of OS may refer to physiological oxidative stress as well as toxic oxidative stress, as outlined in Table 1. Discovering the molecular switch systems and the great development in the field of molecular biology makes OS research attractive (Mitchell et al. 2021; Fernandes et al. 2021; Lorenzen et al. 2016). Also, the advanced development of proteomics, transcriptomics, genomics and nanotechnological techniques changed a lot of OS research concepts (Rosa et al. 2021; Zhang et al. 2021). The present review aims to highlight the recent insights in the field of OS research, demonstrating its merits and pitfalls, illustrating its relation to many diseases and how proteomics and nanotechnology change the research concepts in this field.

Table 1. Simple concepts for oxidative stress.

Category	Term	Reference
Original definition	‘A disturbance in the prooxidant-antioxidant balance in favor of the former’	Yamamoto and Yamashita (1997)
Updated definition	‘An imbalance between oxidants and antioxidants leading to a disruption of redox signaling and control and/or molecular damage’	Touyz et al. (2020)
Specific forms	1. Nutritional oxidative stress: imbalance between oxidant load and antioxidant defense due to inadequate or excess nutrients supplementation.	Sies (1997)
	2. Physiological oxidative stress: imbalance between oxidants and antioxidants due to physiological factors.
	3. Radiation-induced oxidative stress: imbalance between oxidants and antioxidant defense system due to radiation exposure.
	4. Nitrosative stress: biochemical mechanisms of nitric oxide (NO) and superoxide (O2^–) during oxygen metabolism disorders.
	5. Reductive stress: abnormal elevation of reducing equivalents imbalance the oxidation reduction reactions.
Related terms	Oxidative stress, oxidant stress, pro-oxidant stress and oxidative stress status (OSS)	Sies (2015)
Classification	1. Basal oxidative stress (minimal): OS with no detected effect.	Lushchak (2014)
	2. Low-intensity oxidative stress: low imbalance between oxidants-antioxidants system with low cellular effect.
	3. Intermediate intensity oxidative stress: intermediate imbalance between oxidants-antioxidants system with moderate cellular effect.
	4. High-intensity oxidative stress: high imbalance between oxidants-antioxidants system with severe cellular effect.

2. Mechanisms of reactive oxygen production

ROS could be produced by several mechanisms. One of the most important mechanisms for their production is the mitochondrial electron transport chain (ETC). Mitochondrial ETC is considered a potent provider of O2^•- and H₂O₂. Transportation of electrons along with respiratory chain (RC) complexes should end to join O2 to form water while a reduction in O2 contents could lead to the production of ROS (Turrens 2003). Table 2 illustrates the different compartments of mitochondria generated ROS.

Table 2. Different mechanisms for ROS generation in mitochondria.

Respiratory chain complex	Etiology	References
Complex I (NADH dehydrogenase)	Hypoxia, apoptosis	Turrens et al. (1982); Kushnareva et al. (2002)
Complex II (succinate dehydrogenase)	Electron leakage, decrease of electron transfer	Zhang et al. (1998); Lenaz (2001)
Complex III (ubiquinol-cytochrome c reductase)	Ubiquinone supplementation, autoxidation of quinols, interruption of electron flow	Lenaz (2001); Cadenas et al. (1977); Han et al. (2001); Starkov and Fiskum (2001)

Another mechanism for ROS production is induced by neutrophils and phagocytes during their action to fight invaders depending on NADPH oxidases (NOX) (Babior 2004). NOX are membrane-linked enzymes that transfer electrons from NADPH to molecular oxygen. Its role in respiratory burst in phagocytes leads to production of O2^•- (Bedard and Krause 2007). Xanthine oxidase (XO) is also involved in O2^•- and H₂O₂ production during metabolism of xanthine and hypoxanthine (Battelli et al. 2014). COX could be involved in O2^•- production during conversion of arachidonic acid into prostaglandins (Salvemini et al. 2002). Peroxisomes are also involved in ROS during lipid metabolism, where imbalances between ROS and reactive nitrogen species (RNS) production in peroxisomes lead to OS inducing great biomolecular damage (Fransen et al. 2012). Cytochrome P450 enzymes can be imbedded in ROS during their action for catabolism of drugs (Guengerich 2008).

3. The physiological role of oxidative stress

Although OS is a process of imbalance between production and neutralization of ROS, it has been included in various physiological processes. ROS are incorporated in the cell signaling. H₂O₂ for example is acting as a second messenger in signal transduction pathways such as cell differentiation, proliferation, tissue repair and circadian rhythm (Sies 2014). ROS also plays a great role in immune response, as the ROS generated by neutrophils and phagocytes act as defense weapons against invaders (Nathan and Cunningham-Bussel 2013). OS itself is involved in the regulation of various cellular processes like cell growth, differentiation and apoptosis (Finkel and Holbrook 2000). One of its great roles is its involvement in the detoxification of various compounds like drugs and xenobiotics by increasing the activity of cytochrome P450 (Nebert and Vasiliou 2004). On the other hand, NO as member of ROS is able to regulate vascular tone inducing vasodilation and controlling of blood pressure (Forstermann and Sessa 2012). It has been approved that ROS has a great role in neurotransmission processes in the nervous system as it regulates the release of neurotransmitters which approve its role in memory formation and synaptic plasticity (Kishida and Klann 2007).

4. Merits and pitfalls of oxidative stress research

The merits include combining the basics of chemistry related to oxidation-reduction reactions (like electron transfer, free radicals, oxygen metabolites such as H₂O₂, O₂^-, OH, [O], NO free radicals, and NO₃) with biology (Selye 1955). While many kinds of literature use the general term of ROS instead to specify the type of oxidant. According to this aspect, the specific entity of the oxidant should be mentioned, discussed, and analyzed not the generic use of ROS (Weber et al. 1990; Azzi et al. 2004). The concept of ‘one-size-fits-all’ is one of the OS research pitfalls, such as measuring total antioxidant capacity (TAC), which has been used in many pieces of literature, did not give any useful information about the organism exposed to oxidative stress while the specific antioxidant enzyme or the pattern of antioxidants should be measured and analyzed (Pompella et al. 2014).

5. The significance of oxidative stress research

5.1. Molecular redox switches

The concept of molecular redox switches emerged from extensive research into the protein phosphorylation/dephosphorylation processes (Fischer and Krebs 1955). Their significance was, however, fully recognized after discovering the dynamics of cysteine residues of proteins, opening the way for research on redox proteome (Go and Jones 2013; Groitl and Jakob 2014), as a bridge between phosphorylation/dephosphorylation processes and protein cysteine oxidation/reduction, which opens the molecular pathway for signaling cascades as fundamental processes in biology and medicine (Sies 2015). The redox signal transduction, in general, depends on the production of hydrogen peroxides, nitric oxide and hydrogen sulfide, and the reversible redox changes of cysteine moiety of proteins. Moreover, changes in the cysteine residue of proteins can induce several protein modifications which regulate many cellular pathways, such as cell metabolism, cell proliferation, inflammation, migration and gene expression. These changes are controlled by oxidoreductases and protein disulfide isomerases (Lorenzen et al. 2021). Cells are adapted to continual changes in different microenvironments, such as oxygen concentration, nutrients, antigens, immune mediators and stress conditions. In the presence of oxidizing agents (e.g. oxygen) and reducing agents (e.g. cysteine and glutathione) redox sensing and its signal transduction occur and thus affect the physiological status (Sies and Jones 2020). Related to physiological status, higher organisms could use ROS in regulation of multiple physiological processes like: ventilation, erythropoietin synthesis, vascular tone, regulation of oxygen tension, and signal transduction (Droge 2002). Cysteine residues in proteins may act as thiol switches, as their modifications can cause conformational changes of the protein-inducing switches, rendering them less active or inactive (Linzner et al. 2021). Thiol switches allow proteins to rapidly respond to immediate and sudden environmental changes and the underlying mechanism is regulated by the enzymes NOX and protein disulfide isomerase (PDI), while PDIA1 acts as a master regulator of NOX (Fernandes et al. 2021).

5.2. Master switch systems

In recent years, many master switch systems have been discovered, along with the proteins induced, such as catalase, glutathione reductase and alkyl hydroperoxide reductase (Babior 2004). In many bacteria species, OxyR (a regulator of hydrogen peroxide-inducible genes in E. coli and S. typhimurium) is considered a defense system against OS (Storz and Tartaglia 1992). Thus, it is expected that OxyR will be extensively studied in the future to understand stressful and hypoxic conditions that give rise to multiple diseases; this is due to the connection between what happens in mitochondria and E. coli, as many proteins which subjected to S-nitrolysation in E.coli are also S-nitrolysed in mitochondria, so studying the enzymatic regulation of S-nitrolysation in E.coli may give information about the regulatory mechanisms of diseases in mammals (Zhang et al. 2018). Available evidence further indicates that compounds targeting OxyR in bacteria could be used as antipathogenic agents in the fight against these bacteria. It is also well-established that bacterial strains that are deficient in OxyR lose their pathogenicity (Seth et al. 2020). On the other hand, in higher organisms, nuclear factor kappa light chain enhancer of activated B cells (NF$\mathit{\kappa }$B) and nuclear factor erythroid 2-related factor 2 (Nrf2/Keep) are master switches of particular interest (Naito et al. 2021).

Specifically, as NF$\mathit{\kappa }$B is a transcription activating factor of inflammatory mediators like cytokines, it can be induced by many agents, as shown in Figure 2 (Itoh et al. 1997). It could also be a regulator of mitochondrial bioenergetics, morphology and dynamics. Therefore, agents that promote NF$\mathit{\kappa }$B expression could be used to ameliorate mitochondrial respiratory function/morphology (Nennig and Schank 2017). Moreover, as signaling via NF$\mathit{\kappa }$B system is tightly regulated, any dysregulation will lead to disorders (Nisr et al. 2019). Nrf2, on the other hand, is a transcriptional factor that performs a vital function in various physiological conditions like apoptosis, autophagy and clinical conditions, including general disorders related to oxidative stress, diabetes mellitus (DM), cancer, gastrointestinal tract disturbances, neurodegeneration, autoimmune diseases, renal diseases, cardiovascular disorders, respiratory diseases, inflammation, gelatin degradation pathways and fibrosis (Talebi et al. 2021). Hence, going forward, research in this domain should focus on the non-immune functions of such regulatory unit, given that the emphasis is currently given to the pivotal role of NF$\mathit{\kappa }$B in the control and pathogenesis of different types of cancers (Ahmad et al. 2021) and atherosclerosis in humans and animals (Toulassi et al. 2021).

Figure 2. Inducers of NF$\mathit{\kappa }$B signaling; ROS are potent inducers for NF$\mathit{\kappa }$B signaling.

6. Future directions of oxidative stress research

OS is believed to be the main causative agent of many diseases and disorders, as shown in Table 3. The studies in this field are based on diverse hypotheses, some of which are discussed below, along with the role of OS in inflammation, DM and energy hemostasis.

Table 3. List of diseases/disorders linked to oxidative stress.

Disease/disorder	References
Acute respiratory distress syndrome	Marseglia et al. (2019); He et al. (2019).
Aging	Izzo et al. (2021); Mehdi et al. (2021)
Alzheimer disease	Wang et al. (2021); Saeedi and Rashidy-Pour (2021)
Atherosclerosis	Forstermann et al. (2017); Kattoor et al. (2017)
Cancer	Kwiatkowska et al. (2021)
Cardiovascular disease	Fujii (2021)
COVID-19	Lammi and Arnoldi (2021); Francisqueti-Ferron et al. (2021)
Diabetes mellitus	Jha et al. (2016); Eguchi et al. (2021)
Gastrointestinal diseases	Bansod et al. (2021)
Inflammation	Reuter et al. (2010); Sharma et al. (2021)
Inflammatory joint diseases	Feijóo et al. (2010)
Lung diseases	de Genaro et al. (2021)
Neurological diseases	Angelova et al. (2021)
Obesity	Dursun et al. (2016); Grasemann and Holguin (2021)
Parkinson	Vallee et al. (2021)
Pulmonary fibrosis	Phan et al. (2021)
Rheumatoid arthritis	Chakraborty et al. (2021)
Vascular disease	Joffre and Hellman (2021)

6.1. Oxidative stress, endoplasmic stress and autophagy

OS, endoplasmic reticulum stress (ES) and autophagy are interconnected cellular processes. Linking the three processes is of huge importance in the understanding of pathophysiology of various diseases (Sies 2015). Due to the role of endoplasmic reticulum (ER) in protein folding, ES by the way, will lead to misfolded or non-folded proteins. The unfolded protein response (UPR) inside ER is responsible for restoring the ER homeostasis, but prolonged or massive ES can trigger cellular apoptosis (Hetz and Papa 2018). OS and ES are interconnected. At the moment in which ROS induces ES impairing protein folding, conversely UPR activates antioxidants expression to alleviate OS. This approved the intricate link between the two stress responses and their impact on cellular functions (Malhotra and Kaufman 2011; Zito 2015). Autophagy is a process of degradation and recycling of damaged organelles and molecules. Autophagy involves the formation of autophagosomes, which engulf damaged components delivering them to lysosomes. Dysregulation of autophagy is linked with various diseases and metabolic disorders (Mizushima et al. 2008). There is a multifaceted interplay among OS, ES and autophagy. OS itself can induce autophagy as a part of its mechanism to remove cell debris and mitigation of deleterious effects of ROS (Schroder and Kaufman 2005). Similarly, ES can trigger autophagy to degrade the misfolded proteins (Kouroku et al. 2007). In the same time, autophagy plays a crucial role in maintaining ER homeostasis through the removal of damaged proteins and excess ER membranes (Grumati et al. 2017). From our point of view, the interconnection among these three processes is essential for cell survival and homeostasis as their dysregulation is associated with numerous metabolic disorders. Full understanding of these interplay processes may provide us with novel therapies. We recommend further research in this area to unveil the precise molecular mechanisms governing the three processes.

6.2. Oxidative stress and inflammation

OS can trigger many factors which will give rise to an inflammatory condition inside biological cells. This can occur directly (through the release of inflammatory mediators in the blood), or indirectly (through the activation of signals and genes related to the inflammatory conditions). Indeed, OS can trigger many transcription factors, including PPAR-γ, NF-κB, activator protein-1 (AP-1), p53, nuclear factor erythroid 2-related factor 2 (Nrf2) and hypoxia-inducible factor-1α (HIF-1α). The activation of these mediators/factors can lead to the expression of many genes that can code for cytokines, chemokines, cell cycle regulatory molecules, growth factors and anti-inflammatory molecules (Reuter et al. 2010). Under sustained exposure to environmental stress, OS would be able to directly induce DNA and RNA structure changes, lipid peroxidation and carbohydrates and nucleotide changes, or indirectly it would be able to induce cell damage through induction of inflammation, somatic mutations and neoplastic changes (Fang et al. 2009). Cells subjected to inflammation could produce chemical mediators, such as arachidonic acid metabolites, cytokines and chemokines, which can activate cascades of signal transduction and induce modulation of transcription factors, such as NF-κB, HIF-1α, AP-1, signal transducer and activator of transcription 3 (STAT3), nuclear factor of activated T cells (NFAT) and Nrf2, which induce cellular stress responses (Federico et al. 2007; Hussain and Harris 2007), as shown in Figure 3.

Figure 3. Transcription factors modulated by oxidative stress. Nuclear factor kappa B; NF-κB, hypoxia-inducible factor-1α; HIF1-α, activator protein-1; AP-1, Signal transducer and activator of transcription 3; STAT3, Nuclear factor of activated T cells; NFAT and related factor-2; Nrf2, Protein 53; P⁵³, SP1 transcription factor; SP1, Peroxisome proliferator-activated receptor; PPAR.

Recently, new therapies targeting OS cascades have emerged (de Souza et al. 2021), based on the premise that OS is linked to hypertension through high levels of interleukins and TNF-α (Guler et al. 2021), while Nrf2 inhibition will stimulate OS and inflammation (Groitl and Jakob 2014). Extensive research has also been conducted on the host-directed therapy for tuberculosis (TB) by targeting OS markers and studying the OS cascades (Farooqui et al. 2021). Likewise, mesenchymal stem cells derived from bone marrow (BMSCs) are being used to ameliorate OS resulting in liver fibrosis (Groitl and Jakob 2014; Sies and Jones 2020; Amaral et al. 2021; Khadrawy et al. 2021), and ischemic stroke (He et al. 2021). Available evidence further indicates that OS and inflammation could act as mortality predictors in patients undergoing hemodialysis (Sasaki et al. 2021). In this context, it is worth noting extant studies exploring the role of metals and biomolecules as OS triggers or suppressors. For example, zinc has been found to reduce serum inflammation and OS markers like C-reactive proteins (CRP), TNF-α and malondialdehyde (MDA) in adults (Hosseini et al. 2021), while selenium and melatonin have been shown to ameliorate inflammation induced by OS (Sengul et al. 2021; Tu et al. 2021). On the other hand, ammonia (Wang et al. 2021) and uric acid (Deng et al. 2021) have been shown to trigger and exaggerate the development of OS cascades. These findings have led to further research demonstrating that, for example, glutamine is capable of reducing OS and apoptosis induced in bovine mammary epithelial cells (Cheng et al. 2021). In addition, vitamin C (Chaghouri et al. 2021) and vitamin D (Bakhtiari-Dovvombaygi et al. 2021) have been tested for their effects on OS, along with examples of natural products and plant extracts, such as bixin, a carotenoid agent extracted from the plant seeds of the Bixa orellana (Ma et al. 2021), Nigella sativa (Montazeri et al. 2021), and silymarin (Yardım et al. 2021). Moreover, Lobaria pulmonaria and Parmelia caperata, lichen (Zito 2015), ginger extract (Mizushima et al. 2008), Kadsura heteroclita stem ethanol extract (Schroder and Kaufman 2005), sappanone A (Wang et al. 2021), curcumin (Campbell et al. 2021), olive oil extracted from Olea europea L (Mallamaci et al. 2021), grape seed extract (Eid et al. 2021), sesamol (Zhang et al. 2021), pineapple (Seenak et al. 2021) and chitosan oligosaccharides (Lan et al. 2021) have been studied recently for their anti-inflammatory effects.

6.3. Diabetes mellitus as a redox disease

One of the recent theories that illustrate the role of redox impairment in DM is based on the premise that hyperglycemia is a trigger for OS, suggesting that excess glucose can induce redox imbalance (Amorim et al. 2019; Ola 2021; Scheen et al. 2021). This perspective is supported by recent studies indicating that even minor mitochondrial stress (oxidative phosphorylation reaction in mitochondria accounts for the major ROS production) will result in the production of mitochondria-derived ROS (mtROS), whereby failure to compete such ROS will lead to the induction of OS pathways, which will be the etiology of numerous diseases like obesity and insulin resistance (Bjorkman and Pereira 2021). Heavy metals can also contribute to this process, as demonstrated by studies focusing on metal-induced diabetogenic effect (Al Doghaither et al. 2021). OS has also been shown to influence B-cell function through AMP-activated protein kinase (AMPK) activation which will have both useful and harmful effects on the glucose utilization and metabolism (Eguchi et al. 2021), as shown in Figure 4.

Figure 4. Role of oxidative stress in inducing activation of AMPK; ROS activate AMPK pathway inducing either decreasing insulin release when targeting both mTOR and pERK or increasing insulin release when targeting mTOR, NOX, B-cells disallowed genes, miR184, and Ca uptake.

Thus, strategies aimed at counteracting diabetes-induced neurodegeneration involving mitochondrial dysfunction, redox status imbalance, and/or insulin dysregulation seem worthwhile (Carvalho and Cardoso 2021). Additional investigations into OS biomarkers in healthy and diseased subjects are also needed, including the application of recent ‘omic’ technologies (e.g. redox proteomics, lipidomics, metabolomics, and transcriptomics) (Korac et al. 2021). Finally, the impact of heavy metals on the emergence of various diseases should be further examined (Cao et al. 2021).

6.4. Cellular energy balance (energy homeostasis)

The term ‘energy balance’ (homeostasis) describes the adjustment of energy intake and energy expenditure (Keesey and Powley 2008). Thus, dysregulation in this homeostatic process will lead to multiple metabolic disorders like obesity, DM and metabolic syndrome (Nadal et al. 2017). Cells usually have their own mechanisms responsible for maintaining energy homeostasis when exposed to oxidative stress. This could be achieved via fat droplets inside adipocytes which would protect the cell membrane from peroxidation, maintain the saturation of cell membrane, protect the cellular organelles, and ensure the long-term supply of lipids needed for maintaining cell life (Bensaad et al. 2014; Schlaepfer et al. 2015). Failure of homeostasis mechanisms will therefore induce cellular abnormalities and cause imbalance in the cellular energy metabolism. Excess accumulation of fat will lead to obesity, which is the major risk factor for multiple diseases like diabetes, cardiovascular disorders, dyslipidemia, hypertension, metabolic syndrome and fatty liver (Zhou et al. 2021; Colak and Pap 2021). Usually, when cells fail to regulate their energy homeostasis OS plays a significant role in the etiology of this imbalance (Wonisch et al. 2012). The OS stemming from obesity is triggered by many conditions, such as alterations in nutrition, hyperglycemia, hyperlipidemia and chronic inflammation (Manna et al. 2015). Obesity is also associated with high free fatty acids (FFAs) levels in blood. These FFAs will stimulate ROS production through protein kinase C (PKC)-dependent activation of NADPH oxidase (Inoguchi et al. 2000). In addition, the high TAGs levels in obesity are correlated with increased long-chain Acyl-CoA ester levels, which induce inhibition of translocators of mitochondrial adenine nucleotide and lead to adenosine diphosphate (ADP) deficiency, which is a strong initiator and promotor of mitochondrial ROS generation (Bakker et al. 2000). Many enzymes (including SOD, catalase, HO, Prx, and GPx) are capable of activating the defense mechanisms against OS in adipose tissue (Li et al. 2013). However, to ascertain if OS is a consequence or a trigger for obesity, it should be noted that high dietary saturated fats, as well as trans fats and carbohydrates, will increase OS through several chemical pathways, such as synthesis of superoxide via oxidative phosphorylation cascades, glyceraldehyde auto-oxidation pathway, protein kinase C (PKC) activation mechanism, and polyol and hexosamine pathways (Dandona et al. 2010). Nonetheless, OS plays a pivotal role in the development of obesity through the stimulation of preadipocyte proliferation, thus triggering adipocyte differentiation and growth (Higuchi et al. 2013). According to the International Diabetes Federation criteria, metabolic syndrome is diagnosed when three or more of the following features are present: obesity, hypertriglyceridemia, low HDL-cholesterol, hyperglycemia and hypertension (Alberti et al. 2005). OS has played an axial role in the development of metabolic syndrome through deteriorating the insulin production/release and glucose transportation (Hopps et al. 2010). Through lipid peroxidation, as well as protein and DNA oxidation, ROS could damage cell membranes, cellular proteins and DNA. On the other hand, OS can stimulate the release of TNF-$\alpha$ and thus stimulate interleukin-6 (IL-6) production, leading to impaired glucose tolerance, DM and obesity (Stenlöf et al. 2003). Future research endeavors should thus be directed toward the role of oxidant/antioxidant imbalance in the development of cellular energy imbalance and metabolic syndrome.

6.5. Future applications of antioxidants in therapies of oxidative stress disorders

It is well-known today that ROS have a great role for development of various diseases (Table 3). Antioxidants are agents endogenously synthesized or exogenously supplied for neutralization of ROS. Antioxidants work by scavenging ROS, overcome the damaging of biomolecules or modulating the signals inside the affected cells (Valko et al. 2007). In Table 4, various antioxidants, their target and therapies are illustrated.

Table 4. Antioxidants, their targets and future therapies.

Antioxidant	Targets	Current use	Future therapy	References
L-ascorbic acid	ROS scavenger	Dietary supplement	- Liposomal ascorbic acid formulations	Gegotek and Skrzydlewska (2023); Łukawski et al. (2020); Bedhiafi et al. (2023); Gadore et al. (2024)
			- Nanoformulations
Glutathione	Detoxify peroxides	Dietary supplement induces its synthesis with limited clinical applications	- Nanoformulations	Averill-Bates (2023); Minich and Brown (2019); Inal et al. (2023); Wang et al. (2022); Mehrabian et al. (2023)
			- Microcapsules
			- Nanoliposomal formulations
Tocopherols	Lipids peroxides	Dietary supplement	- Cosmeceutical formulations	Kemnic and Coleman (2023); Saleh et al. (2023); Vergara et al. (2023); Thuy et al. (2023)
			- Co-encapsulation formulations
			- Nano-emulsions
N-acetylcysteine (NAC)	- Restoring glutathione	- Acetaminophen over dose	- Liposomal formulations	Deepmala et al. (2015); Alarfaj et al. (2022); Pedre et al. (2021); Singh and Wairkar (2023)
	- Reductant for disulfides	- Drug formulations	- Nanoformulations
	- Scavenger for ROS
Coenzyme Q10 (CoQ10)	Electron transport chain (oxidative phosphorylation)	- Dietary formulation	- Nanoformulations	Arenas-Jal et al. (2020); Pravst et al. (2020); Souza et al. (2023); Zhang et al. (2023); Ergin et al. (2023); Garcia-Becerra et al. (2023)
		- Capsular formulations	- Cocrystal formulations
			- Arginine based proniosomes.
Resveratrol	Activation of antioxidant enzymes	Dietary supplement	- Nanoformulations	Bansod et al. (2021); Sarfraz et al. (2023); Gowd et al. (2022); Badawi (2023); Nasr et al. (2023); Vieira and Conte-Junior (2024); Patole et al. (2023); dos Reis et al. (2023)
			- Microsponge gel
			- Liposomal formulations

7. Application of proteomics and nanotechnology in oxidative stress research

7.1. Oxidative stress and proteomics

Redox proteome refers to the study the post-translational modifications (PTMs) of all proteins exposed to the redox reactions. The thiol redox proteome is a branch of redox proteome focusing on the cysteine thiol redox-based PTMs such as S-sulfenylation (SOH), S-glutathionylation (SSG), S-nitrosylation (SNO) and S-sulfhydration (SSH). The study of such modifications can lead to a better understanding of various aspects of protein functions, such as protein binding capacity, protein activity, conformation, localization and protein interactions with other proteins or molecules (Zhang et al. 2021). The power of proteomics stems from its ability to detect PTMs of a protein after its synthesis which cannot be detected through usual molecular and genetic techniques (Ma et al. 2007). Thus far, about 300 PMTs have been detected in various proteins inside biological cells (Gupta et al. 2014). Most proteins inside cells, when exposed to ROS or any OS, will undergo up- or down-regulation. Proteomics could detect the changes leading to that exposure (Borna et al. 2019). Consequently, proteomics could be applied to compare the unmodified proteins with their modified counterparts in various tissues in health conditions and diseases (Ura et al. 2021). Moreover, proteomics can detect changes due to OS in proteins from metabolic pathways, such as the Krebs cycle and fatty acid metabolism, iron metabolism, and redox proteins (Vasam et al. 2021). Recently, proteomics was utilized to understand how modifications in specific serine and threonine residues of proteins occur. The findings revealed that O-linked N-acetyl-β-D-glucosamine (O-GlcNAc) is repeatedly added and removed by two enzymes O-GlcNAc transferase (OGT) and the O-GlcNAcase (Martinez et al. 2021). Thus, proteomics can be employed to detect oxidative stress biomarkers in multiple injuries and disorders, such as nerve injuries (Zhang et al. 2021), cardiomyopathy (Song et al. 2021), airways inflammation and asthma (Tang et al. 2021), hypertension (Griendling et al. 2021), varicocele (Selvam MK and Agarwal 2021), pulmonary fibrosis (Khan et al. 2021), oxidative changes in breast cancer (Lei et al. 2020) and mechanism of mitochondrial oxidation which lead to many disorders and cell death (Lee et al. 2021). Going forward, identification of valid, specific and precise biomarkers for the detection OS products may assist in better patient management as well as lead to the development of effective therapies against OS-related problems (Gianazza et al. 2021).

7.2. Oxidative stress and nanotechnology

Nanotechnology has found applications in diverse fields, including medicine, industry, technology and agriculture (Giakoumettis and Sgouros 2021). In medicine, it is primarily used to deliver antioxidants for the management, prevention and treatment of OS-related disorders (Bharali and Mousa 2013). Recently, nanoparticles (NPs) were conjugated with SOD to increase its bioavailability and overcome the problems of its rapid clearance as endogenous antioxidant tool to overcome OS (Tee et al. 2016). Similarly, nano films can be used to deliver ferrocenecarboxylic acid (FCA) a generator of hydroxyl free radicals as a tool for improving antitumor therapies through amplified OS (Wang et al. 2021). Enzyme nonreactors could also be constructed to modulate the tumor microenvironment and thus enhance tumor treatment efficacy (Liu et al. 2021). In a recent study, NPs were used to mitigate the effects of OS on skin (Manca et al. 2021; Parekh et al. 2021). In extant studies, different nanomolecules were also used to conjugate multiple biomaterials such as nanocurcumin (Pontes-Quero et al. 2021), oxide NPs (Hsu et al. 2021), nanoencapsulated silymarin (Veisi et al. 2021), graphene oxide nanocomposites (Ahamed et al. 2021), chitosan nanoparticles (Abd El-Hameed et al. 2021), and zinc oxide NPs (Hassanen et al. 2021). The cytotoxicity and genotoxicity of nanoparticles containing different metals has also been studied, including copper oxide (Morsy et al. 2021), bismuth oxide (Ahamed et al. 2021), gold (Aboyewa et al. 2021), titanium dioxide (Tuncsoy and Mese 2021), and iron oxide (Afrasiabi et al. 2021). Future investigations will thus likely focus on developing tools for antioxidant delivery as well as new strategies for overcoming OS, including those based on NPs.

8. Future insight

The future insight is of merging proteomics to elucidate new mechanisms for OS-induced disorders, and applying new nanomaterials to fight OS-related disorders. For clinical aspects, the future will be in discovering new molecular markers for diagnosis and prognosis of OS as etiological agents for disease induction. In addition, research in the OS field will need to be more concise and specific.

9. Conclusion

Further studies are needed to fully benefit from the recent technological advances, including nanotechnology and proteomics. In particular, the mechanisms that link OS with health and disease need to be better understood, and the overall molecular mechanisms of OS should be elucidated further.

Institutional review board statement

There are no experimental methods or use of experimental animals or human subjects.

Acknowledgments

The authors are thankful to the Deanship of Scientific Research at University of Bisha for supporting this work through the Fast-Track Research Support Program.

Author contributions

Conceptualization, Abdelazim MA; software, Abomaghaid MA; validation, Abdelazim MA; resources, Abdelazim MA; writing – original draft preparation, Abdelazim MA; writing – review and editing, Abdelazim MA; visualization, Abomaghaid MA. All authors have read and agreed to the published version of the manuscript.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

Data sharing not applicable – no new data generated.