Redox Pathogenesis in Rheumatic Diseases

Abstract

Despite being some of the most anecdotally well‐known roads to pathogenesis, the mechanisms governing autoimmune rheumatic diseases are not yet fully understood. The overactivation of the cellular immune system and the characteristic development of autoantibodies have been linked to oxidative stress. Typical clinical manifestations, such as joint swelling and deformities and inflammation of the skin and internal organs, have also been connected directly or indirectly to redox mechanisms. The differences in generation and restraint of oxidative stress provide compelling evidence for the broad variety in pathology among rheumatic diseases and explain some of the common triggers and discordant manifestations in these diseases. Growing evidence of redox mechanisms in pathogenesis has provided a broad array of new potential therapeutic targets. Here, we explore the mechanisms by which oxidative stress is generated, explore its roles in autoimmunity and end‐organ damage, and discuss how individual rheumatic diseases exhibit unique features that offer targets for therapeutic interventions.

Introduction

Oxidative stress is a necessary byproduct of the functioning of the innate and adaptive immune systems. ¹ Reactive oxygen species (ROS) allow neutrophils to neutralize invading pathogens. ² ROS generation then results in oxidative stress, the absence of which can cause severe immunodeficiency, such as chronic granulomatous disease (CGD). ³ However, ROS overproduction may overwhelm degradation pathways and thus contribute to the development of rheumatic diseases. ¹ Of note, increased mitochondrial ROS production has been demonstrated in T cells ⁴ , ⁵ and neutrophils of patients with systemic lupus erythematosus (SLE). ⁶ Uncoupled nitric oxide (NO) synthetase reactions and increased NO production by T lymphocytes may also contribute to the accumulation of oxidative stress–generating mitochondria in patients with SLE. ⁷ , ⁸ ROS may operate as a double‐edged sword in T cells, contributing to persistent cellular activation, ⁹ but continuous stimulation also drives T cells into exhaustion. ¹⁰ Alternatively, diminished ROS production by neutrophils may contribute to defective phagocytosis ¹¹ and the accumulation of bacterial DNA capable of activating the innate immune system via toll‐like receptors (TLRs). ¹² , ¹³

Rapid destruction of invading organisms depends on efficient mechanism of ROS production by phagocytic cells. The NADPH oxidase (NOX) and NO synthase enzymes, as well as hydrogen sulfide–producing enzymes and the Fenton reaction, assist in the destruction of phagocytosed bacteria. ¹⁴ Their toxic ROS products destroy bacterial structure and prevent the spread of infection. Due to their highly toxic nature, the production and elimination of ROS are tightly regulated processes. Nuclear factor erythroid 2–related factor (Nrf2) serves as a master regulator of cellular redox homeostasis. ¹⁵ Its main role is thought to coordinate the production of antioxidant moieties, such as glutathione (GSH), thioredoxin, and NADPH, and antioxidant enzymes that neutralize pro‐oxidant factors, such as ROS. Under steady‐state conditions, Nrf2 complexes with Kelch‐like Cullin‐Ring E3 ligase complex‐associated protein 1. ¹⁶ Nrf2 is significantly up‐regulation during times of oxidative stress, namely those triggered by an overproduction of ROS. This then triggers the formation of the antioxidants such as GSH, thioredoxin, and NADPH. ¹⁶ Along this line, Nrf2 deficiency might play a role in systemic inflammatory diseases, such as pneumonia, sepsis, and hepatitis. ¹⁶ Although not considered autoimmune or rheumatic disorders, these inflammatory conditions commonly occur during infection by viruses or bacteria as obvious causative or triggering agents. ¹⁷ During these times of heightened stress on the immune system, it stands to reason that the increased production of ROS, if combined with Nrf2 deficiency, could tip the redox balance into oxidative stress and promotes enhanced inflammation. Here, it is important to note that NRF2‐deficient mice develop lupus ¹⁸ and NRF2 polymorphisms have been associated with the development of nephritis in patients with SLE. ¹⁹

Counterintuitively, hypoxia has recently been posited as another mechanism of oxidative stress generation. Hypoxia‐induced oxidative stress underlies age‐related macular degeneration. ²⁰ The proposed mechanism involves the stabilization of hypoxia‐inducible factors (HIFs), key regulators of cellular adaptation to hypoxic conditions. HIF stabilization aggravates oxidative stress–induced cell death by enhancing iron‐dependent ferroptosis. This form of cell death is mediated via the Fenton reaction, in which H₂O₂ and iron react to generate hydroxyl radicals that trigger lipid peroxidation. In addition, iron transporters involved in non–transferrin‐bound Fe²⁺ import as well as intracellular iron levels can also be up‐regulated. Alternatively, chelation of Fe²⁺ by 2′2‐bipyridyl can completely rescue cells from ferroptosis. ²⁰ Notably, the up‐regulation of HIFs has been implicated in joint and bone destruction in patients with rheumatoid arthritis (RA) ²¹ and osteoarthritis (OA). ²² Moreover, HIF‐1α has been identified as a driver of proinflammatory metabolism via enhancing 2‐hydroxyglutarate production and glycolysis and the expense of mitochondrial tricarboxylic acid metabolism in CD8⁺ T cells. ²³ GSH depletion and deficiency of GSH peroxidase 4 trigger ferroptosis, ²⁴ which may underlie neutropenia in patients with SLE. ²⁵

Mediators of oxidative stress in patients with rheumatic diseases

Controlled production of ROS mediates signal transduction in the innate and adaptive immune systems. ¹ When overproduced or underneutralized, ROS serve as distress signals in all mammalian cells. ROS can spread oxidative stress through the blood stream and cell membranes; ROS are generated when electrons are transferred to molecular oxygen at the inner mitochondrial membrane, thus generating O₂ ⁻ or OH⁻, which can be converted to less‐toxic H₂O₂ by superoxide dismutase (SOD) or neutralized into H₂O by catalase, respectively. ¹ In broad terms, oxidative stress can spread via ROS or highly toxic lipid peroxidation products.

Lipid‐mediated autoimmunity

The lipid‐mediated pathway of autoimmunity may trigger neutrophil cell death via formation of neutrophil extracellular traps (NET‐osis) or NETosis ⁶ , ²⁶ and proinflammatory cell T cell death via necrosis. ⁴ Nonenzymatic lipid peroxidation is primarily mediated by acrolein and 5‐hydroxynonenal (HNE) protein adducts. ²⁷ During oxidative stress conditions, the actions of the eicosanoid–cyclooxygenase—prostaglandin pathway are altered. ⁷ Through a series of arachidonic acid (AA) intermediates, the formation of leukotrienes leads to neutrophil attraction and inflammation. The lipid‐mediated pathway of autoimmunity is also characterized by increased production of eicosanoids and endocannabinoids. ³ ROS also distort the synthesis of anti‐inflammatory prostaglandins and prostacyclins (PGI2s). Additionally, the up‐regulation of eicosanoids increases the differentiation of lymphocytes into T helper (Th) 1, Th2, and Th17 cells. ²⁷ Of particular interest, Th17 cells are an inflammatory subset of CD4⁺ T cells. ²⁸ Their dysregulation can lead to chronic tissue inflammation, causing infiltration of the skin ²⁹ , ³⁰ and kidney by Th17 cells. ³¹ Th17 cells primarily generate antimicrobial peptides and recruit neutrophils via chemokine induction. ²⁸ , ³² HIF‐1α–deficient T cells preferentially develop into regulatory T (Treg) cells over Th17 cells. ³³ HIF‐1α thus plays a critical, proinflammatory role in Th17 differentiation, suggesting another possible mechanism by which hypoxia might contribute to rheumatic diseases, ³² including lupus nephritis. ³⁴

Nonlipid‐mediated autoimmunity

The nonlipid‐mediated path of autoimmunity is a reaction to the imbalance between ROS production and ROS detoxification via GSH or NADPH generated via the pentose phosphate pathway (PPP). ³⁵ GSH is synthetized from three amino acids, glutamine, glycine, and cysteine, the latest serving as the antioxidant moiety. GSH is oxidized by ROS into GSSG, which can be regenerated back into GSH at the expense of NADPH. ³⁵ ROS can also directly oxidize cysteine and methionine residues on proteins. ³⁶ The accumulation of superoxide radicals during hypoxic conditions promotes the formation of H₂O₂ via SOD. The hydroxyl radical generated by the Fenton reaction is responsible for the transformation of phenylalanine and tyrosine compounds. ³⁶ Hypochlorous acid can lead to chlorination of tyrosine, lysine, and histidine residues. ³⁶ The accumulation of chemically modified proteins leads to the generation of neoepitopes. These neoepitopes break tolerance of the immune system to self‐antigens and instead lead to the development of autoreactive T cells and autoantibodies. ² , ⁹ The generation of autoantigens and autoantibodies vary by individual disease and will be discussed later in this paper.

SLE

SLE is an autoimmune rheumatic condition characterized by multisystem organ damage. ³⁷ The manifestations of SLE can be broad and range from simple cutaneous involvement to severe joint, renal, heart, lung, liver, and central nervous system (CNS) complications. The mechanism of SLE pathogenesis involves dysregulated B and T cell development. ³⁸ As stated above, neoepitopes form as the result of chemically modified proteins. In the case of a patient with SLE, B cells produce autoantibodies against these neoepitopes. The autoantibodies produced in patients with SLE are formed against DNA, Sm, and Ro. ³⁹ In response to oxidized DNA, dendritic cells are then activated, triggering the formation of cytokines. ⁶ The presence of antinuclear antibodies is not exclusively diagnostic of SLE. ⁴⁰ However, this marker, combined with key physical manifestations and organ involvement, provides sufficient grounds for diagnosis. To further differentiate from RA, SLE is characterized by decreased apoptosis and increased necrosis of T cells ⁴¹ and NETosis of granulocytes. ⁶ The latter directly triggers the production of type I interferons (IFNs) by dendritic cells in patients with SLE. ⁶ Although IFNα is the IFN most commonly associated with SLE, the role of IFNγ is tied intimately to cell metabolism. ⁴² IFNγ prevents the transcription of inflammatory messenger RNA (mRNA) molecules when complexed with GAPDH. ⁴³ Cells undergoing glycolysis are unable to prevent inflammatory transcription due to the importance of GAPDH in metabolism. This defect is translational and is regulated by the binding of GAPDH to AU‐rich elements within the 3′ untranslated region or UTR of IFNγ mRNA. GAPDH, by engaging/disengaging glycolysis and through fluctuations in its expression, controls effector cytokine production. ⁴⁴ Therefore, cells that undergo constant glycolysis may exhibit increased cytokine production. This may partially explain the metabolic control of CNS involvement in patients with SLE. CNS cells are highly dependent on glycolysis as a source of energy, making them especially vulnerable to any oxidative stress that might arise. ⁴⁵ , ⁴⁶ , ⁴⁷ Preferential use of glucose for glycolysis restricts the ability of neurons to produce NADPH via the PPP, which is critical for maintenance of a reducing environment. ¹ , ³⁵ , ⁴⁷ , ⁴⁸ , ⁴⁹ Of note, IFNγ promotes glycolysis ⁵⁰ and oxidative stress by inhibiting NADPH production via the PPP. ⁵¹ IFNγ is predominantly produced by Th1 cells, which have been associated with nephritis, ⁵² whereas the altered T cell balance overall favors Th2, T follicular helper, T peripheral helper, and Th17 cell development in patients with SLE. ⁵³ , ⁵⁴ CGD is an NOX2‐dependent immunodeficiency that predisposes to infections but also has some SLE‐like features. ⁵⁵ Treatment of CGD with NOX2 activators has been shown to reduce the SLE‐like components of CGD. ¹ Furthermore, in mouse models of pristane‐induced lupus, NOX2 activation was also a successful treatment. ¹ Beyond CGD, autoimmune rheumatic diseases are often linked to the overproduction of ROS, however, it is important to consider the underproduction as well. Just as a proper Th1/Th2 cell balance is necessary to achieve appropriate immune responses, the proper balance of ROS is essential to maintaining a functional immune system.

Oxidative modification may confer antigenicity to host proteins, Ro and La and mitochondrial and nucleosomal DNA. ⁵⁶ Along these lines, lipid peroxides, such as 4‐HNE, can directly increase the antigenicity of Ro. ⁵⁷ Mitochondrial oxidative stress has been originally documented in T cells of patients with SLE and attributed to the accumulation of mitochondria due to diminished mitophagy. ⁵⁸ In turn, diminished mitophagy is mediated by the depletion of mitochondrial fission initiator dynamin‐related protein 1 due to overexpression of Rab4A in lupus T cells. ⁵⁹ Rab4A is encoded by the human T cell leukemia virus–related endogenous sequence 1, an endogenous retroviral locus, which can be activated by environmental triggers, infectious viruses such as HIV‐1 ⁶⁰ or NO. ⁵⁹ Rab4A regulates the trafficking of CD4, CD71, and CD3ζ in human T cells. ⁵⁹ , ⁶⁰ CD71 mediates iron uptake with broad relevance for autoimmune diseases, ⁶¹ including SLE ⁶² (Figure 1). Iron intake may directly enhance kidney injury in patients with lupus nephritis. ⁶⁵ Along with iron overload, the depletion of reduced GSH may contribute to ferroptosis ²⁴ and neutropenia in patients with SLE. ²⁵ Similar to CD4 T cells, ⁵⁹ Rab4A is also activated in the liver of lupus‐prone mice, which is responsive to mechanistic target of rapamycin (mTOR) blockade by rapamycin. ⁶⁶ Along with rapamycin, inhibition of Rab GTPase activity also exerted therapeutic efficacy in lupus‐prone mice. ⁵⁸

Figure 1.

Schematic diagram of metabolic pathways that control oxidative stress in SLE cells. Mitochondrial dysfunction is characterized by blocked electron transport chain activity, elevation of the mitochondrial transmembrane potential (ΔΨm) or mitochondrial hyperpolarization (MHP), and diminished mitochondrial autophagy or mitophagy that underlie the accumulation of oxidative stress–generating mitochondria, ⁵⁸ depletion of ATP, and depletion of reduced glutathione (GSH). ⁴ Oxidative stress in primarily generated at complex I in lupus T cells, which is responsive to treatment with NAC. ⁶³ Mitochondrial oxidative stress may also be supported by increased iron uptake via CD71. ⁶² The resulting oxidative stress activates mTORC1, which promotes glycolysis in CD4 T cells and depletes Treg cells and memory T cells, which depend on mitochondrial ATP production. ⁶⁴ Cytokines may act upstream of mTOR activation in T cells and are subject to therapeutic blockade of receptors such as IFNAR1 by anifrolumab. Metabolic targets of pharmacological interventions are highlighted in green. ΔΨm, elevation of the transmembrane potential; GSH, reduced glutathione; GSSG, oxidized GSH; IL, interleukin; NAC, N‐acetylcysteine; 3‐PEHPC,2‐[3‐pyridinyl]‐1‐hydroxyethylidene‐1,1‐phosphonocarboxylic acid; PPP, pentose phosphate pathway; ROS, reactive oxygen species; Tfh, T follicular helper; Th, T helper; Tph, T peripheral helper; Treg, regulatory T.

Future treatment strategies for patients with SLE should include the prevention of DNA damage. The use of N‐acetylcysteine (NAC), ⁶⁷ mitoquinone, ⁶⁸ and metformin have already been shown to be useful in this regard. ⁶⁹ mTOR forms two complexes that intricately regulate the immune system. Imbalance of mTORC1 or mTORC2 may underlie autoimmunity in patients with SLE. ⁷⁰ , ⁷¹ , ⁷² , ⁷³ Rapamycin is an effective treatment for patients with SLE ⁶⁴ , ⁷⁴ , ⁷⁵ that when combined with NAC, may gain additional efficacy. ⁶⁷

Antiphospholipid syndrome

The production of antiphospholipid antibodies (aPL) is triggered by the antigenicity of β2 glycoprotein I or apolipoprotein H (Apo‐H) due to oxidization of autoepitopes. ⁷⁶ , ⁷⁷ Generation of aPL may be a forerunner of autoantibody formation in patients ⁷⁸ , ⁷⁹ and mice with SLE. ⁶⁶ Apo‐H is secreted by the liver and normally circulates in the bloodstream, where it is protected from oxidization by paraoxonase (PON) 1, which is also secreted by the liver. ⁸⁰ In turn, loss of PON1 leads to oxidization of circulating phospholipids. ⁸¹ , ⁸² As recently uncovered, loss of glycosylation in the liver is a potential cause of diminished PON1 secretion and aPL production, which have been associated with liver inflammation in transaldolase (TAL) deficiency ⁴⁹ , ⁸³ and SLE. ⁶⁶ Interestingly, aPL production has been associated with liver disease ⁸⁴ and oxidative stress and Treg cell depletion in patients with SLE. ⁸⁵ Importantly, mTOR blockade improved the survival of renal transplants in patients with aPL nephropathy. ⁸⁶ mTOR blockade with NAC or rapamycin have also reduced aPL production in patients ⁶⁴ and mice with SLE. ⁶⁶ NAC also blocks endothelial cell damage induced by aPL. ⁸⁷ The production of lipid peroxides, such as 4‐HNE, is also increased in TAL deficiency and responsive to treatment with NAC. ⁸⁸

RA

RA is a systemic autoimmune disease that dominantly involves joints, causing erosive bone loss and chronic deformities. ⁸⁹ Although RA particularly involves the joints, morbidity has been linked to secondary interstitial lung disease (ILD) ⁹⁰ and cardiovascular disease (CVD). ⁸⁹ Oxidative stress has been attributed to deficiency of PON1 in patients with RA and ILD ⁹¹ and CVD. ⁹² Unlike SLE, ⁴⁸ the mechanism of RA is not based on distorted mitochondrial metabolism and oxidative stress but a curious enhancement of PPP activity and greater antioxidant capacity of T cells. ⁹³ Overexpression of glucose 6‐phosphate dehydrogenase accounts for elevated PPP activity and NADPH production in RA T cells. ⁹³ This is apparently coupled with mitochondrial oxidative stress attributed to deficiency of DNA repair nuclease MRE11A. ⁹⁴ Within the joint fluid, deficiency of aspartate may worsen mitochondrial dysfunction and promote tumor necrosis factor (TNF) α production by RA T cells ⁹⁵ (Figure 2).

Figure 2.

RA is characterized by increased mitochondrial oxidative stress that is coupled with enhanced reducing powers due to the overexpression of the PPP enzyme G6PD. Mitochondrial oxidative stress has been ascribed to the deficiency of Asp and low expression of MRE11A in the T cells of patients with RA. ⁹⁵ Asp deficiency may cause enhanced production of TNFα by the T cells in patients with RA. ⁹⁶ In turn, TNFα promotes activation of macrophages, PAD4 expression, protein carbonylation, and NETosis, neutrophil extracellular traps; Asp, aspartate; NET, neutrophil extracellular trap; PAD, peptidylarginine deiminase; PPP, pentose phosphate pathway; RA, rheumatoid arthritis; TNF, tumor necrosis factor.

Patients with RA exhibit a distinct autoantibody profile characterized by autoantibodies to citrullinated proteins and rheumatoid factors (RFs). ⁹⁶ Neutrophil extracellular traps (NETs) play a vital role in exposing these citrullinated proteins to the immune system. ⁹⁷ Unlike patients with SLE, in whom NETs are thought to be a product of oxidative cell death, RA generates NETs via hypoxia and glycolysis. The NETs expose citrullinated proteins, triggering inflammation, and the positive feedback loop continues. NETs in patients with RA are generated by the activity of peptidylarginine deiminase (PAD) 4. PAD4 is responsible for the generation of citrullinated proteins, which are NET attractants. This function is likely linked to the ability of PAD4 to act as an epigenetic modifier via its citrullinating activity. The PADI locus on chromosome 1 is also an independent risk factor for RA. ⁹⁸ The expression of PAD4 on the surface of monocytes and proinflammatory molecules present in the synovium of patients with RA suggests that inflammation attracts NETs, as well as cells expressing PAD4. Moreover, anti–citrullinated protein antibodies (ACPAs) are cross‐reactive to carbamylated and acetylated peptides. In contrast to citrullination, carbamylation is a posttranslational modification associated with oxidative stress. Carbamylation is highly immunogenic, increases the affinity of the ACPA response, and redirects the autoantibodies to PAD enzymes themselves. ⁹⁹ The generation of ACPAs to PAD4 thus creates a feed‐forward loop in patients with RA.

Another key feature of patients with RA is the erosive joint damage. Pathology can be detected in the bone, cartilage, and synovial fluid. The cartilaginous damage in patients with RA may be attributed in part to the elastase effects of NETs. This disrupts the cartilage structure and releases proteins that can be carbamylated and citrullinated. When presented in the context of MHC II, they activate CD4 T cells and generate autoantibodies. Synovial fluid inflammation reflects the participation of T and B cells macrophages, dendritic cells, and neutrophils that synergistically drive autoimmunity to citrullinated proteins. ⁹ As a result of macrophage and osteoclast activation, they release digestive enzymes, including cathepsin K, matrix metalloproteinase (MMP) 9, and tartrate‐resistant acid phosphatase type 5 that degrade the bone matrix. ¹⁰⁰ ROS production is a key activator of osteoclasts, which are responsible for bone erosion in patients with RA. ¹⁰¹ , ¹⁰²

As broadly substantiated, smoking and periodontal disease are just two of the modifiable risk factors of patients with RA. ¹⁰³ , ¹⁰⁴ These risk factors have a strong linkage to oxidative stress and the inflammatory processes. The bone lesions in patients with RA represent a multifactorial inflammation that arises from oxidative stress pathways. Just as in psoriasis (PS), the expansion of Th17 cells leads to excessive inflammation. ¹⁸ However, the Th17 cell–mediated synovitis leads to the generation of osteoclastic signaling molecules such as RANKL and MMPs. ¹⁰⁵ Thus, although not directly involved, oxidative stress can even be linked to bone degradation. In addition to the traditional role of neutrophils as the predominant source of stress‐generating molecules, attention has recently been directed toward the role of fibroblasts. Synovitis in patients with RA is believed to be mediated by fibroblasts, which have destructive subsets that are present and active in patients with RA. ¹⁰⁵

Arguing for a role for antioxidant defenses, activation of NRF2 by dimethyl fumarate can restrain RANKL expression and osteoclast activation. ¹⁰⁶ The traditional mechanism of autoantibody production in patients with RA is the generation of RF, an antibody directed against the Fc region of IgG. ¹⁰⁷ The release of Ig heavy chains from apoptotic B cells may trigger RF production. ¹⁰⁸ Oxidative modification by carbamylation ¹⁰⁹ and lipid peroxides can generate protein adducts with malondialdehyde (MDA) or MDA–acetaldehyde, which has been implicated to trigger RF formation during RA pathogenesis. ¹¹⁰ Antibodies to carbamylated IgG are present in nearly half of patients with RA, including those who are ACPA/RF negative. ¹¹¹ Carbamylation of autoantigens predicts erosive joint damage in patients with RA. ¹¹²

Interleukin (IL)‐23–activated Th17 cells accumulate in germinal centers of secondary lymphatic organs during the prodromal phase of experimental arthritis, where they suppress β‐galactoside α2,6‐sialyltransferase (ST6GAL) 1 expression in differentiating plasmablasts. The consecutive change in the glycosylation profile of IgG elicit a shift toward a proinflammatory autoantibody repertoire and triggers the onset of arthritis. Plasmablasts from patients with RA also display diminished sialyltransferase activity, whereas IgG from these patients show reduced glycosylation profile, as well as increased inflammatory activity, which suggests that related pathways might contribute to the onset and progression of autoantibody‐mediated diseases in humans. Th17 cell–derived cytokines such as IL‐22 seem to promote the suppression of St6gal1 in antibody‐producing cells and can thereby promote a shift to a proinflammatory antibody repertoire. ¹¹³ It appears that the expansion of Th17 cells and IL‐22 production generally associated with oxidative stress ¹¹⁴ , ¹¹⁵ , ¹¹⁶ may reduce ST6GAL1 expression via destabilizing its mRNA. ¹¹⁷ Unlike lupus, deficiency in ferroptosis may underlie disease pathogenesis and contribute to the therapeutic efficacy of TNF blockade in patients with RA. ¹¹⁸ , ¹¹⁹

OA

OA is the most common joint disease in the United States ¹²⁰ and worldwide. ¹²¹ This disease is associated with modifiable risk factors like obesity and nonmodifiable risk factors like age. ¹²⁰ , ¹²¹ However, recent research has linked OA with the loss of NRF2 and the infiltration of inflammatory molecules, cytokines, and ILs, and the severity of OA is also dependent on the degree of inflammatory synovitis. ¹²² NRF2 controls a complex transcriptional/epigenetic and post‐translational network that promotes the activation of antioxidant metabolism. ¹²² Nrf2 inhibits osteoclastogenesis, supports β‐oxidation of fatty acids, and facilitates NADPH regeneration and purine biosynthesis through the PPP during oxidative stress. ¹⁵ Mice deficient in Nrf2 exhibit increased RANKL expression, and bone loss markedly increases, which are reversible by treatment with NAC. ¹²³

PS and psoriatic arthritis

PS is a chronic inflammatory skin condition that presents with erythematous, scaly patches that make up its characteristic appearance. ¹²⁴ Skin patches are dry, itchy, and painful. In addition to causing a significant physical burden, PS can also be a source of social stigma. Thus, the impact of PS is not only physical but mental as well. Of those patients who receive a diagnosis of PS, 25% to 30% will progress to psoriatic arthritis (PsA). ¹²⁵ PsA is an autoimmune disease characterized by a combination of musculoskeletal inflammation and PS plaques. Oxidative stress directly stimulates the proliferation of keratinocytes, a hallmark of psoriatic skin. ¹²⁶ , ¹²⁷ Activation of the PPP has long been demonstrated in human psoriatic skin, presuming its importance in supporting de novo DNA synthesis and protection from oxidative stress. ¹²⁸

Patients with PsA, like those with SLE, can also present with a variety of multisystemic complications, including osteoporosis, uveitis, bowel irritability, and CVD. ¹²⁵ The mechanism of PS and PsA is perhaps better understood by an apoptotic dysregulation mechanism than an oxidative stress mechanism. Genetic predisposition to PsA involves both HLA and non‐HLA genes that partially overlap with those observed in patients with PS skin disease. Microbial dysbiosis and metabolic defects that affect skin integrity and lead to mechanoinflammation are pathognomonic for PS and PsA. Effector cytokines include IL‐23, IL‐17, and TNF, which orchestrate the inflammatory process and serve as therapeutic targets in patients with PS and PsA. With respect to oxidative stress, resistance to cytochrome c–mediated cell death allows for the accumulation of autoreactive T cells and keratinocytes. ¹²⁹ Impairment of cell death in several autoimmune diseases has been linked to cytochrome c release from the mitochondria into the extracellular space and the circulation along with oxidative stress and a failure of activating cell death proteases, such as Apoptotic protease activating factor or APAF and caspases 1, 3, and 9. Redox‐mediated extracellular trap formation by neutrophils and mast cells has been identified as a trigger of IL‐17 release in patients with PS. ¹³⁰ Mast cells also secrete tryptase that mediates mechanoinflammation in patients with PS, accompanied with the release of IL‐6, IL‐8, IL‐17, and IL‐36γ. ¹³¹ Genetic susceptibility to PS has been associated with PSORS1C1 and receptors for IL‐23 ¹³² and IL‐17 ¹³³ (Figure 3). Blockade of IL‐17 or IL‐23 has been found highly effective in treatment of patients with PS ¹³⁴ and PsA. ¹²⁵ Genetic polymorphism of IL‐17RA and IL‐17RC may determine responsiveness to blockade via IL‐23 over IL‐17. ¹³⁵ Most notably, hydrogen‐rich water bath reduced overall levels and peaks of IL‐17, IL‐23, TNFα, CD3⁺, and MDA and expression proliferating cell nuclear antigen in the skin of mice in a model of experimental PS. ¹³⁶

Figure 3.

PS and PsA are orchestrated by a collaboration of adaptive and innate immune system cells. T helper 17 cells of the adaptive immune system sense IL‐23 and secrete IL‐17, and mast cells of the innate immune system secrete tryptase. Mast cells mediate mechanoinflammation and the release of IL‐6, IL‐8, IL‐17, and IL‐36γ. ¹³² Blockade of IL‐17 or IL‐23 has been found to be highly effective in the treatment of patients with PS ¹³⁵ and PsA. ¹²⁶ Hydrogen‐rich antioxidant water bath reduces overall levels and peaks of IL‐17, IL‐23, tumor necrosis factor α, CD3⁺, and malondialdehyde in a model of experimental PS. ¹³⁷ IL, interleukin; PS, psoriasis; PsA, psoriatic arthritis; ROS, reactive oxygen species; TLR, toll‐like receptor.

Sjögren syndrome

Sjögren syndrome (SjS) is characterized by xerostomia and xerophthalmia (dry mouth and dry eye) due to chronic inflammation of the salivary and lacrimal glands. ¹³⁷ Patients with SjS can also experience tooth decay, keratoconjunctivitis sicca, myalgia, arthralgia, and fatigue. Patients with primary SjS have significantly elevated protein carbonyl and advanced oxidation protein products compared to healthy controls ¹³⁷ (Figure 4). Among oxidative protein adducts, protein nitration correlated with disease activity in patients with SjS and SLE. ¹³⁸ Protein nitration also correlated with autoantibody formation in patients with SjS. ¹³⁸ Similar to that in patients with SLE, the generation of oxidative stress leads to the nitration of T cells. ¹³⁸ This then leads to down‐regulation of critical signaling pathways and a lack of check on inflammation. Given the similarities between the mechanism of SLE and SjS, it is highly likely that the nitration of proteins leads to the generation of neoepitopes and the formation of autoantibodies against these neoepitopes. Along these lines, peroxynitrite (PN) modifications can trigger an immune response against carbonic anhydrase (CA) isoenzymes in mice, and PN‐modified CA I and CA II autoantibody titers were found at a significantly high level in both patients with RA and patients with SjS. ¹³⁹ Alternatively, the lacrimal fluid of patients with SjS accumulates oxidized amino acids and lipids, such as hexanoyl‐lysine (HEL) and 4‐HNE. ¹⁴⁰ HEL can attack polyunsaturated fatty acids, including arachidonic acid (AA), and induce the formation of 4‐HNE. ¹⁴⁰ A meta‐analysis of nine articles included 333 patients (628 eye samples) with dry eye disease (DED) and 165 healthy controls (451 eye samples). Seven of nine studies involved patients with established SjS. There is an overall increase in oxidative stress markers in patients with DED compared with healthy controls, with a significant increase in lipid peroxide, myeloperoxidase, NO synthase 3, xanthine oxidase/oxidoreductase, 4‐HNE, MDA, and ROS. Oxidative stress markers were higher in tears, conjunctival cells, and conjunctival biopsies of patients with DED than those of controls.

Figure 4.

Redox‐mediated cellular injury in a patient with Sjögren syndrome (SjS). Corneal and lacrimal gland epithelial cells are sensitive targets for inflammatory oxidative injury that can ultimately lead to corneal ulcer, “corneal melt,” or dry mouth and tooth decay. Inflammation can be restrained by NAC. NAC blocks mitochondrial ROS production and neutralizes lipid hydroperoxides, 4‐HNE, and MDA. HEL, hexanoyl‐lysine; HNE, hydroxynonenal; LOX, lipoxygenase; LTA, lymphotoxin α; LTB, lymphotoxin β; LTC, lymphotoxin γ; LTD, lymphotoxin δ; LTE, lymphotoxin ε; MDA, malondialdehyde; NAC, N‐acetylcysteine; pDC, plasmacytoid dendritic cell; ROS, reactive oxygen species; Tfh, T follicular helper; Th, T helper; Tph, T peripheral helper.

Mitochondrial oxidative stress ¹⁴¹ and decreased expression of antioxidant enzymes have been demonstrated in patients with SjS. ¹⁴² Importantly, treatment with oral NAC in patients with primary or secondary SjS in a double‐blind crossover trial for a four‐week period showed preliminary evidence of therapeutic benefit over placebo. ¹⁴³ Before treatment, there were significantly elevated salivary lactoferrin levels in the patients when compared to 51 healthy controls (P = 0.0005) and significantly decreased levels of tear lysozyme when compared to 24 controls (P = 0.0003). Salivary sodium, potassium, inorganic phosphate, amylase, and IgG, IgA, or IgM levels were not significantly different from control values. After treatment with NAC, patients with SjS reported improvements in ocular soreness (P = 0.004), ocular irritability (P = 0.006), halitosis (P = 0.033), and daytime thirst (P = 0.033). NAC but not placebo improved the van Bijsterveld score (P = 0.026), but neither agent improved the Schirmer test, the tear breakup time or any of the laboratory tests. These results suggest that NAC may have a true therapeutic effect that warrants a follow‐up study of longer duration. Similar to that in patients with SLE, mTOR activation has been demonstrated in patients with SjS. NAC blocks mTOR activation in lupus T cells, ⁶⁷ and rapamycin affords clinical benefit in patients with SLE. ⁶⁴ , ⁷⁴ , ⁷⁵ , ¹⁴⁴ Along these lines, mTOR is activated, ¹⁴⁵ , ¹⁴⁶ , ¹⁴⁷ , ¹⁴⁸ and rapamycin has demonstrated therapeutic benefit in mice ¹⁴⁹ , ¹⁵⁰ and patients with SjS. ¹⁵¹ , ¹⁵²

Scleroderma and progressive systemic sclerosis

Scleroderma or progressive systemic sclerosis (PSS) is a connective tissue disorder characterized by fibroblast activation, extracellular collagen matrix synthesis, skin and organ fibrosis, vascular hyperactivity and remodeling, and autoimmunity. ¹⁵³ , ¹⁵⁴ The vascular injury can present itself in a limited form, composed of calcinosis, Raynaud phenomenon, esophageal dysmotility, sclerodactyly, and telangiectasia, also known as CREST syndrome, that may or may not progress into a systemic form of PSS that can result in organ failure. ¹⁵⁵ Previous studies have reported the strong involvement of genetic and epigenetic factors in patients with PSS ¹⁵⁶ as well as oxidative stress. ¹⁵⁷ Fibroblasts, the principal effector cells, are activated by a proinflammatory milieu of cells and cytokines. Their expansion in parenchymal organs and connective tissues is triggered though TLRs, biomechanical signaling via integrins, hypoxia, and oxidative stress. ¹⁵⁸ PSS skin has an accelerated aging phenotype, with hallmarks of telomere attrition, genomic instability, epigenetic alterations, deregulated nutrient sensing, stem cell exhaustion, and mitochondrial dysfunction. ¹⁵⁹

Expression of the nicotinamide dinucleotide hydrolase ectoenzyme CD38, which is linked to cellular senescence, was up‐regulated in PSS biopsies after adjusting for age. Thus, CD38 expression might promote cellular senescence or senescent cells might induce CD38, connecting NAD metabolism and PSS. ¹⁵⁹ Myofibroblasts are precursors of proliferating fibroblasts in PSS skin, which depend on mTOR activation ¹⁶⁰ , ¹⁶¹ and availability of TGFβ ¹⁶² (Figure 5). NAD may also directly stimulate TGFβ production via CD38. ¹⁶⁴ Accordingly, mTOR blockade may exert therapeutic benefit in patients with PSS ¹⁶⁵ and ILD. ¹⁶⁶ In particular, TGFβ‐activated kinase (TAK) 1 mediates collagen synthesis and myofibroblast differentiation in healthy skin fibroblasts. Of note, treatment with TAK1 inhibitor HS‐276 prevented dermal and pulmonary fibrosis and reduced the expression of profibrotic mediators in bleomycin‐treated mice. ¹⁶⁷ Among antioxidants, NAC showed therapeutic benefit in a subset of patients with PSS ¹⁶⁸ , ¹⁶⁹ and ILD. ¹⁷⁰ , ¹⁷¹ , ¹⁷² Iloprost, a stable analog of PGI2, is an established therapy for skin and pulmonary manifestations in patients with PSS. Human pulmonary microvascular endothelial cells (HPMECs) exhibit increased ROS production and collagen synthesis when treated with sera of patients with PSS. However, sera of patients with PSS who received iloprost treatment failed to increase ROS and collagen synthesis in HPMECs, suggesting a potential antioxidant mechanism of this drug (Figure 5). ¹⁷³

Figure 5.

Redox‐mediated cellular injury in a patient with scleroderma or progressive systemic sclerosis (PSS). Fibroblasts are sensitive targets of pro‐oxidant lipids that are released from inflammatory cells of the adaptive and innate immune system. Iloprost may limit the pro‐oxidant signaling from prostaglandins and lipid hydroperoxides. CD38 was found to reduce NAD⁺ levels and sirtuin activity to augment cellular fibrotic responses, whereas inhibiting CD38 had the opposite effect. Boosting NAD⁺ via NAD⁺ precursor supplementation or genetic or pharmacological CD38 targeting protected mice from skin, lung, and peritoneal fibrosis in mechanistic experiments. ¹⁶³ cADP‐ribose, cyclic ADP‐ribose; DNMQ, 2,3‐dimethoxy‐4‐naphto‐quinone; HIF, hypoxia‐inducible factor; IL, interleukin; SOD, superoxide dismutase; Th, T helper; TLR, toll‐like receptor.

Conclusions

Although oxidative stress is clearly involved in each reviewed rheumatic disease, its roles in pathogenesis and end‐organ resistance require further investigations. Mechanistic studies in patients with PS, PsA, SjS, and PSS/scleroderma overwhelmingly support a proinflammatory role for enhanced ROS production or diminished antioxidant defenses. In contrast, redox signaling may exert opposing roles in patients with SLE and RA, depending on the immune cell type, end organ, body fluid, or intracellular compartment examined. Nevertheless, there are promising signals for clinical efficacy for safely boosting antioxidant defenses, such as enhancing de novo GSH synthesis with NAC administration in patients with SLE or SjS or submerging the skin in H₂‐rich water in patients with PS. ¹³⁷ Further research should focus on the genetic and environmental causes of excess oxidant formation ⁴⁸ and developing a personalized redox‐directed therapeutic approach for each rheumatic disease.

AUTHOR CONTRIBUTIONS

All authors were involved in drafting the article or revising it critically for important intellectual content, and all authors approved the final version to be published. Dr Perl had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.

Study conception and design

Laniak, Perl.

Acquisition of data

Laniak, Perl.

Analysis and interpretation of data

Laniak, Winans, Patel, Park, Perl.

Supporting information

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