Paraoxonase 2 as a target for aging-related diseases

Abstract

Paraoxonase 2 (PON2) is an enzyme exhibiting both lactonase and esterase activities, widely distributed across various tissues and localized within cellular mitochondria. It plays a vital role in innate immunity by restricting bacterial infections and has diverse functions, including the regulation of mitochondrial reactive oxygen species levels and the management of endoplasmic reticulum stress. By alleviating oxidative stress, stabilizing mitochondria, and modulating apoptosis, PON2 emerges as a significant factor in the study of cellular senescence. This review consolidates recent findings regarding PON2's physiological roles, its mechanistic connections to senescence, and the therapeutic potential of modulating its activity. Our analysis highlights PON2's considerable promise as a target for aging-related diseases, including neurodegeneration, metabolic disorders, cardiovascular diseases, chronic inflammation, and cancer.

INTRODUCTION

Cellular senescence is a stable, largely irreversible form of cell-cycle arrest triggered by genomic damage, telomere attrition, oncogenic signaling, or persistent metabolic stress [1-3]. Cellular senescence was initially highlighted for its positive aspects, such as suppressing tumor development, but it actually possesses a double-edged characteristic. While it prevents unchecked proliferation, the chronic accumulation of senescent cells promotes tissue dysfunction through the senescence-associated secretory phenotype (SASP), which includes pro-inflammatory cytokines, chemokines, and proteases [2,4].

A central driver of senescence is mitochondrial dysfunction. Mitochondria, as the primary source of reactive oxygen species (ROS) production during metabolic processes, regulate intracellular energy metabolism, participate in calcium movement and distribution, and influence apoptosis [5-7]. Recent experimental and modeling studies further demonstrated that mitochondrial ROS is not only a byproduct but also a key driver of apoptosis. In cisplatin-induced apoptosis, the mitochondrial ROS-generating pathway plays a central role [8]. Excess ROS leakage impairs bioenergetics, damages macromolecules, and activates pro-senescent signaling cascades [9]. Moreover, the endoplasmic reticulum (ER), which interacts closely with mitochondria, contributes to disease states through ER stress induced by proteostatic failure and maladaptive unfolded protein responses (UPR) in the pro-senescent state [10,11].

Within this landscape, paraoxonase 2 (PON2) has emerged as a versatile intracellular enzyme that buffers oxidative stress and stabilizes mitochondrial and ER function [12-14]. Direct evidence linking PON2 activity to the prevention of cellular senescence, however, remains limited to a small number of experimental studies. Originally identified for its lactonase activity against xenobiotics and bacterial quorum-sensing molecules [15], PON2 has since been shown to play essential roles in maintaining intracellular redox balance [16,17]. Unlike its plasma-associated homologs PON1 and PON3, PON2 is expressed in virtually all tissues and functions within cells, primarily at mitochondria and ER [13,16]. Through these localizations, it regulates the stability of the electron transport chain (ETC) by modulating mitochondrial oxidative stress, calcium homeostasis via ER–mitochondria cross-talk, and ER stress signaling, which is particularly important in senescent cells where ETC activity is significantly diminished and dysregulated [18,19]. This positions PON2 as a potential integrator of metabolic and immune pathways that are broadly relevant to cellular stress responses, including pathways implicated in senescence [20,21].

THE GENERAL FUNCTIONS OF PON2

Enzymatic activity and localization

PON2 has been shown to exhibit potent lactonase activity, with a particular affinity for acyl-homoserine lactones (AHLs), quorum-sensing molecules produced by Gram-negative bacteria [15], as well as other structurally related cyclic lactones [22]. Through hydrolysis of these substrates, PON2 interferes with bacterial communication and limits host exposure to exogenous lactone-containing compounds.

Homocysteine (Hcy) is an intermediate generated during methionine metabolism [23] and can be converted intracellularly to Hcy-thiolactone by methionyl-tRNA synthetase, leading to protein N-homocysteinylation and cytotoxicity [24-26] (Fig. 1). While PON1 has been shown to hydrolyze Hcy -thiolactone, there is currently no direct biochemical or structural evidence demonstrating that PON2 catalyzes this reaction. Although PON2 displays broad lactonase activity toward multiple aliphatic and aromatic lactones, including γ-butyrolactone and bacterial AHLs [15,22], its capacity to hydrolyze endogenous mammalian lactone metabolites such as Hcy-thiolactone has not been experimentally established.

Fig. 1. Overview of intracellular methionine–homocysteine (Hcy) metabolism.

Dietary protein–derived methionine is metabolized to Hcy through the S-adenosylmethionine (AdoMet) and S-adenosylhomocysteine (AdoHcy) pathway. Intracellular Hcy is converted to Hcy-thiolactone, which reacts with protein lysine residues to generate N-Hcy-proteins and Nε-Hcy-Lys. Hcy can also be converted to cysteine, leading to the formation of S-Hcy-proteins. PON2 is indicated as a regulator of the interconversion between Hcy and Hcy-thiolactone. PON2, paraoxonase 2.

Structural insights into PON2 remain limited and are largely inferred from homology with other paraoxonase family members. The precise configuration of its active site and its relevance to endogenous substrate recognition therefore remain unresolved. Clarifying the structural determinants of PON2 substrate specificity will be essential for understanding whether its lactonase activity extends beyond antibacterial functions and for evaluating its relevance to intracellular metabolic homeostasis.

PON2 is strictly intracellular, enriched at the inner mitochondrial membrane and ER [12,13]. At mitochondria, PON2 stabilizes ETC function, especially at complex III, limiting electron leakage and ROS formation [14,16]. At the ER, PON2 contributes to redox buffering and protein folding capacity, thereby safeguarding proteostasis [13,17]. These localizations enable PON2 to protect two of the most stress-sensitive organelles, mitochondria and ER, integrating metabolic balance and protein quality control (Fig. 2).

Fig. 2. Overview of PON2 functions.

Schematic illustration of diverse functions of PON2 categorized into three groups. In the ‘Enzymatic Activity and Localization’ category, PON2 has lactonase activity, detoxification activity, and it is enriched in the ER and mitochondria. In the ‘Redox Homeostasis and Apoptosis’ category, PON2 optimizes ETC and regulates cytochrome c release. In the ‘Innate Immunity and Host-Pathogen Interaction’ category, PON2 has antimicrobial effects, linked to an immune feedback circuit, and can suppress ROS accumulation. PON2, paraoxonase 2; ER, endoplasmic reticulum; ETC, electron transport chain; ROS, reactive oxygen species.

Redox homeostasis and apoptosis

Mitochondria produce ROS during normal respiration. However, if these are produced in excess, they can damage the mitochondria's membranes, proteins and nucleic acids [7,9]. PON2 lowers basal ROS output, preserving mitochondrial membrane potential and ATP generation by optimizing ETC function [14,16]. Its activity also prevents lipid peroxidation of membranes [27].

Another physiological function of PON2 is to directly modulate apoptosis. Stress-induced mitochondrial dysfunction activates intrinsic apoptotic pathways via cytochrome c release and caspase activation. The overexpression of PON2 suppresses this cascade, thereby reducing the activity of caspase-3 and -7 and improving cell survival [13,16]. In parallel, PON2 regulates ER stress responses, dampening protein kinase R (PKR)-like ER kinase (PERK), inositol-requiring enzyme 1 (IRE1), and activating transcription factor 6 (ATF6) activation [10,17]. Together, these actions are consistent with a broad cytoprotective role of PON2 across endothelial, neuronal, and hepatic cell types (Fig. 2).

Despite extensive evidence supporting the cytoprotective roles of PON2, its direct endogenous substrates and molecular targets within mitochondria and ER remain poorly defined. Consequently, it remains unclear whether PON2 exerts its intracellular effects through direct enzymatic reactions, modulation of local redox microenvironments, or protein–protein interactions. This mechanistic uncertainty represents a major gap in the current understanding of PON2 biology and limits definitive interpretation of its roles in senescence and aging-related pathologies.

Innate immunity and host–pathogen interaction

PON2’s lactonase activity is not only biochemically interesting but also physiologically significant in the context of innate immunity. By hydrolyzing bacterial AHLs, PON2 disrupts quorum-sensing communication systems used by Gram-negative pathogens such as Pseudomonas aeruginosa to coordinate virulence and biofilm formation [15,28]. This 'quorum-quenching' capacity is linked to the innate immune system, which limits bacterial persistence and reduces the host's susceptibility to infection. This represents an evolutionary conserved antimicrobial defense [29,30].

Animal studies further underscore this role. PON2-deficient mice exhibit increased severity of P. aeruginosa infection, with enhanced biofilm development and impaired clearance [30,31]. In infection models, endogenous PON2 activity contributes to improved host survival by attenuating bacterial quorum sensing and limiting disease severity in infected hosts [30].

Importantly, PON2 also modulates the host immune response during infection. By suppressing mitochondrial ROS accumulation, it limits tissue damage that results from excessive immune activation [32]. Macrophages lacking PON2 release higher levels of pro-inflammatory cytokines and a reduced ability to adopt an M2 anti-inflammatory phenotype [33]. Thus, PON2 offers dual protection by directly neutralizing bacterial quorum signals and by indirectly restraining host-driven oxidative and inflammatory responses.

PON2 AND CELLULAR SENESCENCE

Mitochondrial protection

Mitochondrial impairment is a hallmark of senescence, as excess ROS and calcium overload trigger apoptosis and energy decline [7,9]. PON2, localized at the inner mitochondrial membrane, reduces electron leakage at complex III [14,16]. Separately, excessive calcium influx and premature opening of the mitochondrial permeability transition pore have been implicated in the induction of cellular senescence in multiple experimental systems [34,35]. Whether PON2 directly modulates senescence through calcium-dependent mitochondrial permeability remains untested.

Furthermore, PON2 has been reported to influence pathways associated with mitochondrial quality control, including autophagy-related processes and mitochondrial morphology [20,36,37]. Direct evidence demonstrating that PON2 activates mitophagy in senescent cells is currently lacking. Cells overexpressing PON2 preserve mitochondrial function by lowering mitochondrial superoxide and increasing respiratory chain activity [17,38]. These roles of preventing injury and clearing damaged organelles reinforce the importance of PON2 in mitochondrial quality control [16,20] (Fig. 3).

Fig. 3. PON2-mediated modulation of cellular senescence.

Schematic illustration of the role of PON2 in cellular senescence. In the ‘Mitochondrial Protection’ category, PON2 regulates calcium uptake, and activates mitophagy. In the ‘ER Stress Modulation’ category, PON2 reduces ER ROS, and stabilizes calcium lipid transfer between the ER and mitochondria. In the ‘Inflammatory Regulation’ category, PON2 reduces SASP. PON2, paraoxonase 2; ER, endoplasmic reticulum; ROS, reactive oxygen species; SASP, senescence-associated secretory phenotype.

ER stress modulation

Persistent ER stress is a potent inducer of senescence, primarily through the signaling of a maladaptive UPR [39]. PON2 reduces ER ROS accumulation, thereby attenuating PERK, IRE1, and ATF6 activation [13,17]. This modulation enhances protein-folding capacity and limits proteotoxic stress [10,40,41]. While chronic ER stress is a recognized driver of senescence, whether PON2-mediated attenuation of UPR signaling directly suppresses senescence has not been experimentally demonstrated.

As the ER and mitochondria are closely connected via calcium and lipid transfer, the dual localization of PON2 enables it to stabilize communication between the two organelles [42]. By buffering redox balance at both sites, PON2 may limit propagation of stress signals that can contribute to senescence-associated phenotypes, reinforcing its potential role as a coordinator of organelle resilience [14,43] (Fig. 3).

Inflammatory regulation

Senescent cells amplify inflammation via the SASP, driven by NF-κB activation and persistent cytokine secretion [44,45]. PON2 is suggested to mitigate inflammatory responses by modulating NF-κB signaling and attenuating expression of pro-inflammatory cytokines such as TNF-α and IL-6 [46,47]. In immune and vascular cells, PON2 deficiency results in exaggerated cytokine release, thereby amplifying inflammatory signaling that may promote senescence propagation [33].

Overexpression of PON2, by contrast, favors anti-inflammatory states, including macrophage M2 polarization [33]. By modulating oxidative stress, ER stress, and inflammatory signaling, PON2 may influence pathways that are broadly implicated in age-associated inflammation [19,29]. However, a direct role for PON2 in limiting inflammaging in vivo has not yet been established (Fig. 3).

OUTCOMES OF PON2 MODULATION

Effects of PON2 overexpression

Elevated expression of PON2 exerts widespread benefits across cellular systems, largely due to its ability to fine-tune both mitochondrial bioenergetics and redox homeostasis. In mitochondria, PON2 enhances the efficiency of the ETC, where electron leakage is a major source of superoxide radicals [16]. By stabilizing complex interactions and minimizing ROS escape, PON2 not only prevents lipid peroxidation and mitochondrial DNA damage but also sustains ATP generation under high metabolic demand [14,27]. This effect leads to enhanced energy metabolism in tissues that are metabolically active, such as the heart, liver, and skeletal muscle.

At the ER, PON2 overexpression has been shown to suppress maladaptive UPR signaling under sustained stress conditions, preventing the activation of pro-apoptotic transcription factors and caspase [17]. This action has direct consequences for cellular proteostasis, especially in tissues where protein synthesis and folding are highly dynamic, such as hepatocytes and pancreatic β-cells.

The benefits of PON2 overexpression are not confined to the cellular level. In the vascular system, it protects endothelial integrity by dampening vascular inflammation, and reducing the risk of foam cell formation [19,48]. In the central nervous system, increased expression of PON2 has been associated with reduced oxidative stress and improved neuronal survival in experimental models [49]. Whether these protective effects translate into meaningful disease modification in neurodegenerative disorders remains unknown.

Effects of PON2 deficiency

By contrast, loss of PON2 initiates a cascade of maladaptive responses that compromise cellular homeostasis at multiple levels. At the mitochondrial level, deficiency results in inefficient electron transfer, excessive ROS generation, and progressive disruption of membrane potential [16]. This contributes to release of cytochrome c and activation of caspase-dependent apoptotic pathways [14]. Without PON2's stabilizing influence, mitochondria become more vulnerable to calcium overload. This leads to the opening of the permeability transition pore and necrotic cell death.

In the ER, absence of PON2 exacerbates stress signaling through the PERK, IRE1, and ATF6 branches of the UPR, leading to heightened cytokine production and reduced protein-folding efficiency [17]. Moreover, the failure of PON2 to mitigate ER stress allows persistent UPR activation to spill over into mitochondrial dysfunction via Ca²⁺ and ROS-mediated cross-talk, thereby compromising bioenergetic integrity [18].

Systemically, PON2-null mice provide further insights into the broad impact of deficiency. PON2-null mice exhibit impaired glucose tolerance, elevated insulin resistance, and lipid accumulation in hepatic tissue [20,50]. Retinal degeneration models confirm that highly energy-dependent cells are more vulnerable to oxidative injury in the absence of PON2 [34]. Moreover, immune cells from PON2 deficient animals secrete higher levels of pro-inflammatory cytokines, amplifying systemic inflammation [33]. In summary, PON2 deficiency may accelerate the cellular decline associated with aging and predisposes individuals to a wide array of pathologies driven by mitochondrial dysfunction, ER stress and inflammatory dysregulation (Fig. 4).

Fig. 4. Comparative effects of PON2 overexpression and deficiency in mitochondria, ER, and systemic physiology.

Schematic comparison of the effects of PON2 overexpression and PON2 deficiency across three biological levels including mitochondria, ER, and systemic physiology. At mitochondrial level, there are different efficiency of ETC. Thus, there are resulting in altered ROS and ATP production. At ER level, there are altered protein expressions, including cytokine, and pro-apoptotic transcription factors. At systemic physiological level, PON2 modulation affects multiple organs and tissue homeostasis. PON2, paraoxonase 2; ER, endoplasmic reticulum; ETC, electron transport chain; ROS, reactive oxygen species; UPR, unfolded protein responses; NAFLD, non-alcoholic fatty liver disease; NASH, non-alcoholic steatohepatitis.

DISEASE TARGETS FOR PON2-BASED THERAPEUTICS

Relatively little is known about how PON2 expression and activity are regulated during aging. Existing studies suggest that oxidative stress, inflammatory signaling, and metabolic cues can influence PON2 expression in specific disease contexts, such as atherosclerosis and cancer [51-54]. However, whether age-associated changes in transcriptional regulation, post-translational modification, or subcellular localization contribute to altered PON2 function remains largely unexplored. Addressing these regulatory aspects will be critical for understanding how PON2 activity is modulated across the aging trajectory. Against this regulatory uncertainty, the following sections summarize disease contexts in which PON2-associated cellular processes have been investigated most extensively.

Neurodegenerative disorders

Neurodegenerative diseases represent compelling contexts for PON2-targeted therapy. In Parkinson’s disease, dopaminergic neurons in the substantia nigra are especially vulnerable due to high oxidative metabolism and dopamine auto-oxidation.

Experimental studies in dopaminergic neuron models indicate that PON2 reduces mitochondrial ROS accumulation and supports cellular bioenergetic capacity under oxidative or toxic stress conditions [55]. In line with these findings, recent work in Parkinson’s disease–relevant experimental systems suggests that PON2 activity contributes to dopaminergic neuron resilience rather than directly preventing neurodegeneration [56]. However, no study has yet demonstrated that modulation of PON2 alters disease progression or neuronal loss in in vivo Parkinson’s disease models.

In Alzheimer’s disease contexts, evidence is even more indirect. PON2 has been shown to act as an intracellular mitochondrial antioxidant in neuronal models, limiting oxidative injury and cellular stress responses [49,57,58]. Given that mitochondrial dysfunction and ER stress are central features of Alzheimer’s disease pathology [59], these observations suggest a potential relevance of PON2-mediated redox regulation to neuronal vulnerability [49]. However, no experimental study has directly linked PON2 activity to amyloid- or tau-driven neurodegenerative processes, nor has PON2 been shown to exert disease-modifying effects in Alzheimer’s disease models.

Overall, available data support a model in which PON2 modulates intracellular stress tolerance in neurons by regulating mitochondrial and ER redox homeostasis [49]. While such functions may be relevant to neurodegenerative disease biology, current evidence supports an associative and mechanistic plausibility framework rather than a causal or therapeutic role. Future studies employing disease-relevant in vivo models will be essential to determine whether PON2 modulation can meaningfully influence neurodegenerative disease onset or progression.

Cardiovascular disease

Atherosclerosis and ischemic heart disease are strongly linked to oxidative injury within vascular tissues. PON2 protects endothelial cells by reducing ROS formation, maintaining nitric oxide availability, and limiting leukocyte adhesion [13,14,60]. It is evident from the research carried out on macrophages that PON2 plays a pivotal role in the promotion of M2 polarization. This in turn leads to a reduction in plaque vulnerability and an inflammatory burden [14,33,61]. Human atherosclerotic plaques consistently show decreased PON2 expression, supporting its relevance to human disease [51]. By preserving mitochondrial integrity in cardiomyocytes, PON2 may also reduce ischemia-reperfusion injury, a major cause of cardiac dysfunction following myocardial infarction [62]. Thus, the enhancement of PON2 activity holds promise as a strategy for both the prevention and mitigation of cardiovascular disease.

Metabolic syndrome and type 2 diabetes

In metabolic syndrome, PON2 safeguards cellular metabolism by enhancing oxidative phosphorylation, reducing lipid peroxidation, and preventing mitochondrial dysfunction in liver. Deficiency or genetic variation of PON2 impairs hepatic insulin signaling and β-cell function, suggesting a contributory role in the development of insulin resistance [21,63]. Pharmacological activation of PON2 with compounds such as vutiglabridin has demonstrated improvements in autophagy, mitochondrial function, and lipid handling in models of non-alcoholic steatohepatitis [36]. These results indicate that therapeutic activation of PON2 may offer a novel metabolic intervention strategy that is complementary to standard antidiabetic and lipid-lowering therapies.

Cancer

The role of PON2 in cancer is multifaceted, as it can act as a facilitator for the survival of tumors [64] and concurrently present therapeutic opportunities. In the context of tumors, elevated metabolic demand and hypoxia have been observed to induce oxidative stress. This, in turn, has been shown to promote mitochondrial efficiency and prevent apoptosis, with the upregulation of PON2 [17,65]. This property confers resistance not only to intrinsic stressors but also to chemotherapeutic agents that rely on oxidative cytotoxicity. Targeted inhibition of PON2 in cancer cells could therefore enhance treatment efficacy [65,66]. Nonetheless, systemic inhibition carries potential risks given PON2's role in protecting non-malignant tissues [57]. Future strategies may involve tumor-specific delivery systems or small molecules that selectively disrupt PON2 function in malignant contexts while preserving its beneficial effects elsewhere [17].

Chronic inflammatory diseases

Persistent inflammation underlies chronic inflammatory disorders such as rheumatoid arthritis and inflammatory bowel disease [67,68]. While PON2 has been implicated in modulating inflammatory environments, its role appears to be context-dependent and mechanistically indirect. In endothelial systems, PON2 regulates tissue factor–mediated coagulation activation under stress conditions [47], thereby influencing vascular inflammatory–thrombotic responses rather than directly inhibiting canonical inflammatory signaling pathways such as NF-κB or inflammasome activation. In macrophages, PON2 deficiency is associated with increased release of pro-inflammatory cytokines, contributing to immune dysregulation [33]. Consequently, modulation of PON2 activity may represent a potential immunometabolic strategy for influencing oxidative and inflammatory axes implicated in chronic inflammatory disorders.

CONCLUSION AND FUTURE PERSPECTIVES

This review describes PON2 not as a disease-specific effector but as a modulator of cellular stress responses, operating at the intersection of mitochondrial redox regulation, ER homeostasis, and inflammatory management. PON2 doesn't seem to work through a single dominant pathway, which is a sign of functional decline that happens with age [1-4,6,7]. Instead, it seems to affect several cellular vulnerability nodes that become less stable as we get older. This systems-level positioning sets PON2 apart from conventional disease drivers, suggesting that its relevance in aging-related diseases primarily relates to enhancing cellular resistance under chronic metabolic and proteostatic stress [3,9].

Current evidence does not support a reductionist perspective of PON2 as a direct suppressor of cellular senescence or as a standalone "anti-aging" factor. A large number of experimental investigations indicate that PON2 affects upstream stress environments but do not conclusively demonstrate causal control over the initiation, progression, or tissue-level aging phenotypes of senescence [12-14,16-18,39]. This differentiation is crucial, as it redefines PON2 from a presumed senescence regulator to a context-dependent modulator, whose functional impacts are likely to vary according to cell type, stress intensity, and disease stage, in accordance with current models of senescence heterogeneity and stress-induced aging [3,35,39].

PON2-related studies are presently constrained by various conceptual and methodological limitations. First, PON2 biology remains mechanistically under-resolved: aside from its lactonase activity toward bacterial AHLs, its endogenous mammalian substrates, interaction partners, and the relative contribution of catalytic versus non-catalytic functions remain incompletely defined [15,16,19,57]. Second, regulatory aspects of PON2—how its expression, subcellular localization, or activity are altered during aging—have received limited systematic attention, despite evidence that oxidative stress, inflammatory signaling, and metabolic perturbations can modulate PON2 expression in disease contexts [19,29,48,51]. Third, translational inference is inadequate, as extensive human genetic studies and longitudinal clinical data directly associating PON2 variation with aging-related disease susceptibility or progression are limited and predominantly correlative [50,51,63].

To rectify these limitations, future studies need to switch from mere descriptive correlations to causal testing paradigms, incorporating cell type–specific modulation of PON2 in disease-relevant in vivo models and direct examination of senescence endpoints instead of surrogate stress indicators [34,35,39]. Parallel efforts should aim to define how PON2 regulation changes across the aging trajectory by integrating transcriptional, post-translational, and spatial control mechanisms, an approach increasingly recognized as essential for understanding age-associated network failure [19,41]. Finally, anchoring PON2 biology in human data—through population genetics, tissue-based expression analyses, and biomarker development—will be essential to determine whether PON2 modulation has predictive or therapeutic relevance in aging-related diseases [51,63,67].

In summary, PON2 is a node in the cellular stress networks that cause diseases related to aging. It is biologically plausible but not fully understood. It will be important for the future of aging biology to find out if PON2 is a permissive resilience factor, a disease modifier, or a context-dependent liability. We can only accurately define PON2's role and responsibly assess its therapeutic potential through a combination of mechanistic and translational approaches.