Aspects of xenobiotics and their receptors in stroke

Abstract

Stroke is devastating and the second leading cause of disability and death worldwide. The pathophysiology of stroke is intricate involving oxidative stress, ionic imbalance, and excitotoxicity leading to cell death. The current therapeutic strategies for ischemic stroke primarily aim to restore cerebral blood flow by removing clots using intravenous thrombolysis and mechanical thrombectomy. However, hemorrhagic stroke requires different therapeutic interventions, where intravenous thrombolysis worsens the persistent condition. Nevertheless, the present treatment strategies do not provide effective neuroprotection as they have limitations such as narrow time window, specialized clinics and personnel, and higher expense. Therefore, studies on novel therapeutic strategies that can render neuroprotection over an extended time with minimum adverse effects are solicited. Xenobiotics are agents that are foreign to the biological system but can regulate their metabolism by binding to different xenobiotic receptors (XRs) to produce toxic substances. Modulation of XRs in different preclinical studies have shown benefits in the stroke outcome. Therefore, targeting XRs may be a future therapeutic strategy for stroke intervention. The present review briefly discusses various implications of xenobiotics and their receptors to evolve as a potential therapeutic target for prospective use as an adjunctive therapy for stroke.

Highlights

• Xenobiotic receptors have role in regulating neuronal health in various neurological disorders.

• Exposure to xenobiotics deteriorate stroke outcome.

• Targeting xenobiotic receptors may be promising for stroke intervention.

Aspects of Xenobiotics and their receptors in Stroke: Stroke leads togeneration of excessive free radical mediated oxidative stress. Different xenobiotics such as superoxides, PAH, and so on can exacerbate this condition and lead to poor stroke outcome. Exposure to xenobiotics can also lead to blood–brain barrier breakdown and activation of various xenobiotic receptors that can increase apoptotic cell death. However, different chemical xenobiotics that are intended to act as drug interventions can render neuroprotection via alleviating neuroinflammation following stroke. (Image adapted from Servier Medical Art by Servier which is licensed under a Creative Commons Attribution 3.0 Unported License; https://smart.servier.com/)

1. INTRODUCTION

Xenobiotics are the chemical agents that are considered foreign bodies to organisms and are extrinsic to their normal metabolism. ¹ This includes pesticides, environmental contaminants, occupational chemicals, foreign chemicals created by other organisms (such as histamine present in bee sting), drugs, and medications. There are several routes of xenobiotics entry to a biological system, such as respiratory tract (inhalation), dermal contact, gastrointestinal tract (ingestion), and accidental exposure by parenteral route. ¹ Continuous exposure and accumulation of a high amount of xenobiotic are toxic. Therefore, they need to be metabolized to facilitate faster excretion and minimize toxicity. Xenobiotics undergo one or more of four metabolic fates such as unchanged elimination, unchanged retention, spontaneous chemical transformation, and enzymatic metabolism. In general, hydrophilic xenobiotics tend to be eliminated unchanged via urine, whereas lipophilic compounds prefer fecal elimination. Lead, cadmium, and several other inorganic substances most likely get retained unchanged within the body for long time that may lead to various deleterious biological effects. Organic xenobiotics get stored in the adipose tissue. Most of the xenobiotics do not get biotransformed by spontaneous chemical transformation, while enzymatic metabolism predominates their biotransformation. ¹

Stroke is one of the leading cause of morbidity and mortality worldwide, while according to recent studies, case fatality may rise in coming years. ² The reason behind this increase is multifaceted; however, different modifiable and non‐modifiable risk factors play a pivotal role in stroke prevalence. ² Broadly, stroke is categorized as hemorrhagic (13%) and ischemic (87%). Both categories are influenced by a similar kind of risk factors; however, some differences can be noticed among their etiologies, ² for instance, hypertension is an important modifiable risk factor for hemorrhagic stroke. It may contribute to atherosclerosis and, later on, to ischemic stroke. ²

In developing countries, where a high burden of hypertensive disorders is present, the prevalence of hemorrhagic stroke is reported. ² However, people with increased Western style diet and lifestyle are becoming more susceptible to ischemic stroke. ² This epidemiological transition creates the basis to study the influence of lifestyle hazards that may have an important role in stroke susceptibility. In this scenario, various particulate matters present in polluted air reported as an important xenobiotics that may complicate stroke etiology. ² Therefore, the current review focuses on various aspects of xenobiotics and their receptors in stroke. To prepare this review, a thorough literature search was performed using PubMed, ScienceDirect, EMBASE, and Google Scholar using different keywords, such as xenobiotics, stroke, biotransformation of xenobiotics, xenobiotic receptors (XRs), and xenosensors. Studies describing involvement of various xenobiotics in stroke occurrence and prognosis were included. Various in vitro and in vivo preclinical studies that directly or indirectly relate xenobiotic and stroke are considered in this review.

2. CROSS‐TALK BETWEEN XENOBIOTICS AND STROKE

Xenobiotics can be categorized as exogenous and endogenous according to their sources. ³ The exogenous xenobiotics include various chemical compounds of foreign origin such as drugs, environmental pollutants, food additives, hydrocarbons, pesticides, synthetic polymers, and oil mixtures. ³ Endogenous xenobiotics are metabolites such as bile acid, bilirubin, and various hormones that may exert toxic effects similar to exogenous xenobiotics. ³ These can be further classified as weak, recalcitrant, and persistent according to their stability in the environment owing to their structural organization, which helps resist microbial biodegradation. ³ The recalcitrance of a xenobiotic compound typically depends on the type, number, and position of chemical bonds, which contribute to their xenobiotic characteristic. ⁴ According to fate, xenobiotics can also be classified as degradable and nonbiodegradable. Degradable xenobiotics such as vinyl chloride, toluene and pentachlorophenol can be biotransformed or cometabolized, while nonbiodegradable xenobiotics such as dichlorodiphenyltrichloroethane (DDT) and ethidium bromide (EtBr) are highly toxic and may participate in detrimental biogeochemical changes. ³

About three‐quarters of global stroke can be prevented by modifying behavioral risk factors that are related to lifestyle hazard (such as smoking, poor diet, and low physical activity). In 2016, a study first identified and reported in Lancet Neurology that environmental air pollution is an emerging risk factor of stroke worldwide. ⁵ According to the study, air pollution and environmental risk related to ambient exposure of fine particulate matter <2.5 μm (PM_2.5) contributed to 16.9% of stroke‐related disability‐adjusted life‐years (DALYs). Overall, DALYs related to air pollution and tobacco smoke (both active and passive) were 29.2% and 22.8%, respectively. ⁵ In a multipollutant model, exposure to sulfur dioxide (SO₂) along with particulate matter is reported to increase stroke mortality up to 7% even in a city with low air pollution. ⁶ Long‐term exposure to air pollution particularly rich in nitrogen dioxide (NO₂) in a region with European health‐based air quality has also reported increased ischemic stroke susceptibility. ⁷ Earlier, the pathogenic progression of ischemic stroke has been reported to be significantly related with air pollution that results in increased mortality. ⁸ Apart from air pollution, persistent organic pollutants (POPs) are reported to result atherosclerosis along with thromboembolism, which ultimately leads to stroke. A study using the database from the New York Statewide Planning and Research Cooperative System (SPARCS) database of stroke‐related hospital admission reported that POP exposure is an important risk factor of cerebrovascular disease. ⁹ Lead toxicity is another important xenobiotic‐related hazard that can exacerbate stroke outcomes. The World Health Organization (WHO) reports a 5.65% of global stroke burden in 2019 is accounted to lead toxicity alone. ¹⁰

In tropical countries, infections associated with pathogens (xenobiotic) are closely related to stroke prevalence. ¹¹ Cerebral malaria, Chagas disease, and neurocysticercosis are the major central nervous system infections that may predispose transient ischemic attack (TIA). ¹¹ , ¹² Bacteria such as tuberculosis, syphilis, and brucellosis and viruses such as arenavirus, flavivirus, dengue fever virus, Japanese encephalitis, Nipah virus, human immunodeficiency virus (HIV), and so on, are various pathogens that can lead to stroke complications. ¹¹ Recently, several studies also reported increasing clotting problems that resulted in a sudden rise of stroke mortality following Ebola and SARS‐CoV2 infection. ¹¹ Cerebral infarction due to neurocysticercosis ranges from 2% to 12%; however, extensive epidemiological studies are required to infer a robust relationship between tropical infection pathogens and associated stroke risk. ¹¹ A summary of different xenobiotics and their effects on stroke has been tabulated in Table 1.

Table 1.

Different xenobiotics and their effects in stroke

Xenobiotic	Effect on stroke	Reference
PM2.5	Enhanced stroke‐related DALYs	Feigin et al. 5
SO2	Can increase stroke mortality up to 7% in a city with low air pollution	Amancio and Nascimento 6
NO2	Can increase ischemic stroke susceptibility to normal individuals	Avellaneda‐Gómez et al. 7
POPs	Results in atherosclerosis and thromboembolism, leading to stroke	Shcherbatykh et al. 9
Pathogen‐mediated CNS disorders
Cerebral malaria, Chagas disease, and neurocysticercosis	Predispose TIA	Reis et al. 11
		Huang et al. 12
Tuberculosis, syphilis, and brucellosis	High stroke risk	Reis et al. 11
Arenavirus, flavivirus, dengue fever virus, Japanese encephalitis, and Nipah virus, HIV, Ebola, and SARS‐CoV2	High stroke risk	Reis et al. 11
Drugs, dietary supplements, and alcohol	Metabolites may act on various endogenous proteins, leading to vascular complications	Hillbom and Numminen 13
Metals
Arsenic	Predisposes atherosclerosis	Reis et al. 11
Cadmium	Can initiate and progress atherosclerosis, hypertension, and kidney damage, leading to stroke	Reis et al. 11
Lead	Exacerbates stroke outcome. According to WHO reports a 5.65% of global stroke burden in 2019 is accounted to lead toxicity	Vongsfak et al. 10
PAHs	Atherosclerosis	Bang et al. 14
Tunicamycin, thapsigargin, brefeldin A, and DTT	Predispose cell death following ER stress	Lafleur et al. 15
Synthetic glucocorticoids	Can be transported through the blood–brain and blood placental barrier, leading to exacerbation of stroke outcomes	Craft 16
		Balkaya et al. 17

Abbreviations: DALYs, disability‐adjusted life‐years; DTT, dithiothreitol; HIV, human immunodeficiency virus; NO₂, nitrogen dioxide; PAHs, polycyclic aromatic hydrocarbons; PM2.5, fine particulate matter <2.5 μm; POPs, persistent organic pollutants; SO₂, sulfur dioxide; TIA, transient ischemic attack; WHO: World Health Organization.

3. BIOTRANSFORMATION OF XENOBIOTICS

Continuous accumulation of various xenobiotics can lead to lethal effects. Therefore, these agents must be biotransformed to facilitate their excretion from the body to alleviate the toxic effects. ¹⁸ The biotransformation of xenobiotics includes several chemical changes in the parent compounds to alter their toxicological profile. ¹⁸ Hepatocytes are the major sites of xenobiotics biotransformation as they are rich in endoplasmic reticulum, which helps in optimum production of important enzymes that participate in biotransformation. ¹⁹ Accumulation of xenobiotics can activate XRs, which play a crucial role in catalyzing various immune reactions. ²⁰ XRs regulate the expression of several phase I and phase II drug‐metabolizing enzymes and transporter proteins, as well as coordinating with the hepatic defense network against xenobiotic threats. ²⁰ Phase I of biotransformation involves enzymes such as cytochrome P450 oxidases, which biotransform xenobiotics through oxidation, reduction, or hydrolysis. ²¹ , ²² Phases II and III comprise glutathione S‐transferases that catalyze the conjugation of reactive electrophiles with glutathione and removal of GSH conjugates by transporters, leading to cellular protection ²¹ , ²³ (Figure 1).

Figure 1.

Role of xenobiotics and their possible mechanism of action on exacerbating stroke outcome: Different xenobiotics can enter to the biological system via inhalation, ingestion, and dermal contact. Following entry, they undergo biotransformation by phase I, II, and III reactions in hepatocytes with the help of various enzymes. The metabolic by‐products and their parent form further act on XRs that finally lead to exacerbation of stroke outcomes via excessive apoptosis. XRs, xenobiotic receptors. UPR, unfolded protein response. Source: Image adapted from Servier Medical Art by Servier which is licensed under a Creative Commons Attribution 3.0 Unported License; https://smart.servier.com/and https://biorender.com/.

The biotransformation of xenobiotics can also be regulated by various factors that control generation of different biotransformational by‐products. The active concentration of accumulated xenobiotics is one of the major chemical factors that modulate the rate and route of enzymatic modification. ²⁴ Some xenobiotics may activate particular signaling cascade, which enhances their metabolism, whereas other xenobiotics can inhibit their metabolism, which results in toxic effects. ²⁴ The chemical nature such as the polarity of the xenobiotics also regulates their biotransformation via modulating their absorption capacity, which affects the rate of elimination from the body. ²¹ Xenobiotic interaction with various proteins also affects their clearance from the host. Apart from these, various environmental factors, like temperature, light, moisture, and ionizing radiation, may affect the rate of xenobiotics biotransformation. Different physiological states such as pregnancy, inflammation, infection, and fasting can play a pivotal role in the regulation of xenobiotic biotransformation as these factors can modulate the intrinsic enzyme activity, which regulates the rate of biotransformation. ²⁵ Drugs, dietary supplements, and alcohol can also act as xenobiotics as they are external to the human body. Following biotransformation of these, the metabolites may act on various endogenous proteins that may result in vascular complications. According to research, a large amount of alcohol consumption can aggravate stroke risk. Alcohol contributes to high blood pressure, which may lead to hemorrhagic stroke. It can also result in atrial fibrillation, which is a major risk factor of cardio‐embolic stroke. Alcohol can also interrupt blood clotting, leading to hemorrhagic and ischemic stroke complications. ¹³ Metals are another important xenobiotics that precipitate atherosclerotic and blood clotting‐related complications, leading to stroke. Arsenic is reported to produce atherosclerosis, which is a well‐known risk factor of ischemic stroke. ¹¹ On the other hand, cadmium can initiate and progress atherosclerosis, hypertension, and kidney damage, which lead to stroke. ¹¹

Epoxide hydrolases (EHs) are involved in xenobiotic detoxification and endogenous epoxide biotransformation. ²⁶ They catalyze the hydrolytic conversion of highly reactive epoxides to less reactive diols. As a result, EHs coordinate critical signaling pathways that maintain cell homeostasis. ²⁷ EH plays a key role in xenobiotic metabolism due to its broad range of substrate selectivity and ubiquitous expression in the liver and other major organs. ²⁷ Antagonizing EH enzymes by administering 4‐[[trans‐4‐[[(tricyclo[3.3.1.13,7] dec‐1‐ylamino) carbonyl] amino] cyclohexyl] oxy]‐benzoic acid (tAUCB) is reported to exhibit a beneficial effect in a transient middle cerebral artery occlusion (tMCAo) model of cerebral ischemia, implicating EH as a novel therapeutic target in stroke. ²⁸ Microsomal and soluble epoxide hydrolase (sEH) are the two mammalian enzymes that play a vital role in xenobiotic metabolism. ²⁷ , ²⁹ In stroke pathophysiology, there is an elevation in sEH levels, which complements an mEH spectrum of substrates that further increase biotransformation of xenobiotics, leading to the production of potentially harmful substitutes. ²⁷ , ³⁰ mEh inactivates reactive epoxides by converting them into diols, making them crucial components against epoxide like‐xenobiotics. ³⁰

4. XENOBIOTICS AFFECTING STROKE PATHOPHYSIOLOGY

Continuous exposure to xenobiotics may deteriorate the outcome of ischemic stroke. ³¹ These can result in mitochondrial dysfunction, increase oxidative stress, disrupt blood–brain barrier (BBB), and increase apoptosis and autophagy. ²¹ , ³² , ³³ Following stroke, the gastrointestinal mucosal flora gets considerably altered, which modifies the host immune system, leading to infection and alteration of the membrane transport system as well as xenobiotic degradation pathways. ³⁴ The exact mechanism of how different xenobiotics can influence the predisposition and outcome of stroke is lesser known. Nevertheless, according to previous studies, exposure to xenobiotics post‐stroke can regulate enzyme metabolism, increase oxidative stress, and impair the balance between apoptosis and autophagy, which aggravate stroke‐mediated BBB disruption. ²⁷ , ³⁵ , ³⁶ , ³⁷

4.1. Alteration of BBB permeability by xenobiotics in stroke

The BBB controls the transport various endogenous and exogenous xenobiotics and their metabolites from the bloodstream to the brain. A collection of different transport proteins are expressed on the luminal (blood‐facing) and abluminal (brain‐facing) plasma membranes of endothelial cells from its basic foundation. ³⁸ Some of these transporters enhance the permeability of the barrier to the necessary nutrients, while others selectively block xenobiotics from entering. ³⁸ Poststroke BBB disruption allows xenobiotic entry to the brain, which exacerbates cellular damage and contributes to cognitive impairment. ³² Following stroke, the BBB gets compromised, which leads to excessive accumulation of xenobiotics in the brain that may exacerbate brain damage. ³⁵ A key component of the BBB is P‐glycoprotein, which is an ATP‐driven efflux transporter that regulates the transport of drugs into the brain according to its expression level, selectivity, location at the luminal membrane, and high transport potency. ³⁹ Several studies have reported that the expression of P‐glycoprotein in the BBB is crucial in regulating xenobiotics entry to the brain from the systemic circulation. ³⁹ The expression of P‐glycoprotein in the BBB is higher in stroke, which is reported to alter sensitivity toward xenobiotics. ³⁹ However, the mechanism of this altered sensitivity is still unclear. ³⁵ In another study, activation of the XRs such as constitutive androgen receptor (CAR) is reported to increase expression of efflux pumps in the BBB, which can limit the xenobiotic entry into the brain. ⁴⁰ However, reasons to what extent CAR and XR can alter the permeability of the BBB for xenobiotics is yet to be explored.

4.2. Alleviation of oxidative stress following xenobiotic exposure in stroke

Biotransformed metabolites are not necessarily toxic in all cases. Drugs are the exogenous xenobiotics that are intended to alleviate existing pathology. In the case of stroke, generation of reactive oxygen species (ROS) and other oxidative species play a pivotal role in exacerbating neurological damage. ⁴¹ ROS produced due to lipid peroxidation, protein oxidation, and DNA oxidation leads to oxidative stress generation, which results in neuronal damage, cell death, and secondary brain damage following stroke. ⁴¹ , ⁴² , ⁴³ The Kelch‐like ECH‐associated protein 1–nuclear factor (erythroid‐derived 2)‐like 2 (Keap1–Nrf2) pathway is considered a significant signaling cascade in protecting the cells from oxidative and xenobiotic stresses (Figure 2). ⁴⁴ Superoxide generated by polycyclic aromatic hydrocarbons (PAHs) results in oxidative stress generation via continuous conversion of diones (quinones) to hydroquinones. ³⁶ In mammals, Nrf2 (encoded by Nfe2l2 gene) is a transcription factor that acts as an antioxidant and regulates cellular redox homeostasis as well as phase II detoxification reaction of xenobiotics. ⁴⁵ Nrf2 acts as cytoprotective via binding to the antioxidant response element (ARE) following heteromerization with the musculoaponeurotic fibrosarcoma (Maf) protein, which further regulates a battery of cytoprotective genes, including glutamate–cysteine ligase (GCL), thioredoxin reductase 1 (Txnrd1), NAD(P)H‐quinone oxidoreductase 1 (NQO1), and heme‐oxygenase‐1 (HO1). ⁴⁵ This chain of events alters the oxidative state of cells and results in cellular protection. Following xenobiotic exposure, Nrf2 overexpression has been observed to upregulate the expression NQO1 gene. ⁴⁵ , ⁴⁶ Keap1 is a necessary protein involved in the regulation of this pathway, which ubiquitinates Nrf2 in the cytoplasm under normal conditions. ⁴⁴ Following ubiquitination, Keap1 gets detached from Nrf2, which further gets stabilized when exposed to ROS. As a result, Nrf2 translocates into the nucleus and promotes the transcription of different cytoprotective genes. Various experimental animal models of ischemic stroke have demonstrated the beneficial role of the Nrf2/ARE antioxidant signaling pathway following exposure to different drugs (xenobiotics). ⁴⁴ Dimethyl fumarate and monomethyl fumarate were reported to protect against acute brain damage and post‐stroke edema through anti‐inflammatory cytokines (interleukin [IL]‐10 and IL‐12 p70) related to the active Nrf2 pathway. ⁴⁷ The Nrf2 activator sulforaphane pretreatment in the tMCAo model was reported to prevent the BBB breakdown via upregulation of HO1 expression. ⁴⁸ The Nrf2 inducer tert‐butylhydroquinone is reported to reduce cortical damage and sensorimotor deficits up to one month in a mice model of ischemic stroke. ⁴⁹ Trans‐resveratrol is reported as neuroprotective via upregulation of Nrf2 expression and mitochondrial antioxidants. ⁵⁰ Apart from these, carbon monoxide, tetramethyl pyrazine, Korean red ginseng, and curcumin are also reported to induce the Nrf2 signaling pathway and have exhibited beneficial effects in the experimental stroke models. ⁵¹

Figure 2.

Role of different XRs following stroke: Following stroke, xenobiotics exposure and oxidative stress can be detrimental via activation of RXR and PXR, whereas activation of the PPAR exhibits neuroprotective activity by inhibiting apoptosis and reducing ROS generation. ARE, antioxidant response element; BBB, blood–brain barrier; PPAR, peroxisome proliferator‐activated receptors; PPRE, peroxisome proliferator response element; PXR, pregnane X receptor; PXRE, pregnane X response element; ROS, reactive oxygen species; RXR, retinoid X receptor; XRs, xenobiotic receptors. Source: Image adapted from Servier Medical Art by Servier which is licensed under a Creative Commons Attribution 3.0 Unported License; https://smart.servier.com/and https://biorender.com/.

4.3. Cell death regulation by xenobiotics in stroke

Cerebral ischemia causes two types of apoptosis: the intrinsic mechanism, which is triggered by mitochondrial cytochrome c release and caspase 3 stimulation, and the extrinsic mechanism, which is triggered by activation of cell surface death receptors and caspase 8 stimulation. ⁵² Apoptosis is one of the major programmed cell death pathways where XRs have emerged as promising mediators. ⁵³ , ⁵⁴ One important XR is peroxisome proliferator‐activated receptors (PPARs), whose agonists such as troglitazone, rosiglitazone, and pioglitazone have been reported neuroprotective in different CNS disorders, including cerebral ischemia. ³⁷ The PPAR forms a heterodimer with retinoid X receptors (RXRs) to regulate various transcriptional genes, resulting in activation of the neuroprotective pathway. ³⁷ However, xenobiotics promote apoptosis via inhibiting the PPAR activation pathway, which can further deteriorate the stroke outcome. ³⁷ Apart from these, estrogen, progesterone, and mineralocorticoid XRs can mediate antiapoptotic signaling, whereas androgen and glucocorticoid receptors (GRs) are proapoptotic mediators. Estrogen receptor‐mediated antiapoptotic activity is one of the important factor of poststroke neuroprotection, Whereas progesterone receptor‐mediated antiapoptotic activity is particularly important in traumatic brain injury, spinal cord injury, and ischemic stroke. ³⁷

The endoplasmic reticulum (ER) is the major site for xenobiotic biotransformation; therefore, parent xenobiotics and their metabolite can influence normal endoplasmic reticulum function. ¹⁵ Xenobiotics can block protein transfer from the endoplasmic reticulum to Golgi bodies, resulting in pooling of proteins in the endoplasmic reticulum. Therefore, new protein‐folding mechanism gets perturbed, which results in endoplasmic reticulum stress and unfolded protein response (UPR) activation. Prolonged UPR pushes cells toward apoptosis. ¹⁵ Various xenobiotics, like tunicamycin, thapsigargin, brefeldin A, and dithiothreitol (DTT), are well known to predispose cell death following endoplasmic reticulum stress. ¹⁵

Apart from apoptotic cell death, stroke also results in ferroptosis, which is directly related to iron and amino acid metabolism. ⁵⁵ , ⁵⁶ Following ischemic stroke, iron accumulation increases in the brain via compromised BBB, which results in excessive ROS production by Fenton's reaction. ⁵⁷ Increased expression of glutathione peroxidase (GPX) following xenobiotic exposure (carvacrol and several monoterpenic phenols) may rescue post‐stroke hippocampal neurons via alleviating ferroptosis. ⁵⁸ According to Lanet et al., ⁵⁹ Naotaifang, a compound in Chinese medicine, can limit neuronal ferroptosis following ischemic stroke by modulating TFR1/DMT1 and SCL7A11/GPX4 pathways. In hemorrhagic stroke, the occurrence of ferroptosis is inversely related to the expression of Nrf2. Nrf2 promotes GSH and GPX expressions, resulting in an enhanced antioxidant function. Selenium, another xenobiotic can also promote GPX expression and result in the alleviation of ferroptosis. ⁵⁸ Parthanatos is another type of cell death pathway that results due to oxidative damage of DNA, leading to overactivation of poly(ADP‐ribose) polymerase (PARP). Poly(ADP‐ribose) polymers induces apoptosis‐inducing factors that push cells toward death.

4.4. Xenobiotic preconditioning

Xenobiotic‐induced preconditioning in stroke is known as “pharmacological preconditioning,” which is referred to as increased tissue tolerance in long‐term ischemic injury. ⁶⁰ It may limit cell death and infarct size and improve organ function. It is an effective neuroprotective method against stroke that can be attained by various agents. ⁶¹ Preconditioning with isoflurane is reported to induce iNOS‐dependent neuroprotection in rat pups following neonatal stroke. ⁶² An ATP‐sensitive potassium channel opener, Bimakalim, can exert the ischemic preconditioning‐like effect following myocardial ischemia, which further results in reduced infarct size. ⁶³ Apart from this, various inflammatory cytokines and metabolic inhibitors can be exogenously delivered as a chemical preconditioning agent. ⁶⁴

5. IMPLICATIONS OF DIFFERENT XRS IN STROKE

XRs are reported to play crucial roles in regulating neuronal health in various neurological pathologies, including stroke. ⁶⁵ These receptors lack any physiologically relevant endogenous ligand. ⁶⁵ Among various XRs, the constitutive androstane receptor (CAR), pregnane X receptor (PXR), PPARs, and AhR have been subjected to substantial research owing to their dominance in controlling hepatic responses to medicines and environmental chemicals. ²¹ CAR and PXR are categorized in the nuclear receptor (NR) family and are also known as NR1i3 and NR1i2, respectively. ⁶⁵ XRs regulate the transcription of drug‐metabolizing enzymes and transporters to break down and eliminate foreign particles from the body. ⁶⁵ , ⁶⁶

5.1. Pregnane X receptor

PXR and CAR have been extensively found in vasculature, where they regulate different vascular functions. ⁶⁷ PXR, also known as the steroid X receptor (SXR) and pregnane‐activated receptor (PAR), is activated by a wide range of chemicals with no obvious structural similarities, including endobiotics like steroid hormone metabolites, vitamins, and bile acids, and xenobiotics like herbals, macrolide antibiotics, and antifungals. ²¹ PXR has been shown to target genes involved in xenobiotic metabolism (phase I and phase II) and efflux in the liver and gut. As a result, PXR is regarded as a “master regulator” of xenobiotic defense at the cellular and molecular levels. ²¹ Progesterone has been reported neuroprotective in various animal models of neurodegeneration, stroke, and traumatic brain injury. ³⁷ An 18‐kDa novel translocator protein ligand ZBD‐2 can effectively prevent NMDA‐induced excitotoxicity and apoptosis in a mouse model of cerebral ischemia via pregnenolone and progesterone synthesis, which can be sensed by the PXR. ⁶⁸ It may act as a master switch in stroke recovery and is reported to regulate NF‐κβ signaling and to maintain intestinal barrier integrity, which may affect stroke recovery. ⁶⁹ The single‐nucleotide polymorphisms (SNPs) in the PXR gene also play an important role in ischemic stroke outcomes. ⁷⁰ A 1‐year clinical study on platelet function along with genotyped SNPs of the PXR and cytochrome p450 was conducted on 634 stroke patients who were on antiplatelet medication. The study reports that SNP of a PXR variant rs13059232 can be an independent risk factor for patients in clopidogrel medication. However, the result was not similar with patients on aspirin medication. Therefore, rs13059232 can serve as preliminary evidence of a platelet function biomarker for poststroke clopidogrel therapy. ⁷⁰ The PXR is reported to express in the brain and modulate neuronal plasticity. It involves clearance of cholesterol‐based steroids and neurosteroids. ⁷¹ It is an upstream factor for neuro‐steroidogenesis, which has a major role in the brain and behavioral plasticity. ⁷¹ The mineralocorticoid receptor (MR) and GR are reported to exert opposite roles in modulating synaptic plasticity, ⁷² whereas both can impair hippocampal synaptic plasticity and acquisition of related memories. ⁷²

5.2. Constitutive androstane receptors

CAR is reported to play a pivotal role in bile acid metabolism, regulation of cholesterol level, and lipid and glucose homeostasis. ⁶⁷ It is also a major regulator in cellular proliferation and cancer. ⁶⁵ CAR activation can occur either through direct ligand binding within the receptor's ligand‐binding pocket or through the least defined indirect activation pathways. ⁷³ Both of these mechanisms of interaction cause the receptor to be released from a cytoplasmic tethering complex, allowing it to proceed to nuclear translocation, dimerization with its RXR—a nuclear partner, and binding of the receptor dimer to requisite DNA motifs associated with CAR‐inducible genes. ⁷³ To date, studies related to the CAR and its role in stroke regulation are limited. In a study involving the metabolism of thyroid hormone following middle cerebral artery occlusion in a rat model of ischemic stroke, the CAR level has been positively corelated with ischemia/reperfusion injury and hypothyroidism. ⁷⁴

5.3. Aryl hydrocarbon receptors (AhRs)

AhR signaling pathway activation is the primary molecular defense of body in response to xenobiotics. ⁶⁷ Unlike the CAR and PXR, which have been limitedly studied, for almost 40 years, the AhR has been investigated and reported to be involved in several cellular processes. PAHs such as 3‐methylcholanthrene (3‐MC) and halogenated aromatic hydrocarbons (HAHs), such as 2,3,7,8‐tetrachlorodibenzo‐p‐dioxin (TCDD), are common xenobiotic activators (ligands) of the AhR due to their high affinity toward the receptor. Atherosclerosis is a well‐reported risk factor of ischemic stroke. ¹⁴ AhR activation can lead to the activation of macrophages, dysfunction of vascular endothelia following an increase in adhesion molecules (ICAM‐1 and VCAM‐1), and generation of ROS, which is associated with atherosclerosis progression. ⁶⁷ Activation of neural cell‐specific AhRs following ischemic stroke results in increased astrogliosis and decreased neurogenesis. Hence, the inhibition of the AhR can preserve neurogenesis and act as a promising therapeutic strategy. ⁷⁵ In a mouse model of middle cerebral artery occlusion‐mediated cerebral ischemia, the tryptophan metabolite l‐kynurenine can act as endogenous AhR agonist, which participates in poststroke neuronal impairment. ⁷⁶ Apart from these, the downstream genes of AhR (CYP1A1, GSTT1, and GSTM1) serve important roles in stroke outcomes. ⁷⁷ A case–control study with 353 stroke patients reports a large number of CYP1A1 gene 3′‐flanking regions (T6235C) compared with 376 controls. The gene–gene interaction study also showed increased cerebral infarction risk in patients without GSTM1 but with CYP1A1c genotype. ⁷⁸ Another study in the South Indian population has reported a significant association of CYP1A1cc genotype with stroke outcomes, whereas in a study conducted in the Chinese population, the said association reported opposite effects. ⁷⁹ , ⁸⁰ Therefore, further studies to investigate the role of the AhR in stroke must consider population‐wise genetic variations.

5.4. PPARγ and GRs in stroke

PPARγ is an another conventional XR whose agonist can exert a neuroprotective effect in various neurological pathologies, including stroke. ³⁷ An RXR agonist bexarotene is reported to be beneficial in a mice model of cerebral ischemia as the RXR remains as a heterodimer with PPARγ. ⁸¹ The major mechanism of PPARγ‐mediated post‐stroke neuroprotection is due to the inhibition of apoptosis and attenuation of ischemia‐mediated ROS generation. ³⁷ , ⁸² It can also regulate various inflammatory cytokines to modulate vascular inflammation. Various PPARγ agonists can cross the BBB and exert post‐stroke neuroprotection. ⁸²

Synthetic glucocorticoids are qualified as prime example of xenobiotics. It has a wide spectrum of potencies and pharmacological characteristics. This is due to their relative and absolute affinity for the GRs, as well as their sensitivity for and accessibility of their associated metabolic enzymes, as well as their capacity to interact with and modify glucocorticoid‐responsive genes. ⁸³ Synthetic glucocorticoids can pass the BBB and the placenta because they are extremely lipophilic. Certain evidence states that there may be a steady release of glucocorticoids during and after stroke. The animals express a greater GR in the most susceptible parts of the brain to ischemia damage, and persistent exposure to high glucocorticoid levels after stroke may render them affected by glucocorticoid‐induced ischemic cell death. ¹⁶ A study on 129/SV mice reports that the GR antagonist can help in the treatment of chronic stress‐induced stroke resulted due to endothelial dysfunction. ¹⁷

5.5. Estrogen receptors in stroke

Estrogen has been shown to provide a comprehensive regulation of neurons, including plasticity and neuroprotection in stroke. Anoxia, oxidative stress, glutamic acid, hydrogen peroxide, iron, and Aβ peptide are a few of the assaults that they protect neurons from. ³⁷ However, the use of estrogens as a neuroprotective agent in humans has a number of drawbacks, mostly owing to the molecule's endocrine effect on peripheral tissues and that lead to an elevated risk of hormone‐dependent stroke. ³⁷ Estrogen also has a role in neuronal plasticity and can help neurons survive following stroke. Mainly, it acts via estrogen receptor (ER) alpha, which can be activated by DNA demethylation following ischemic damage. ⁸⁴ As a result, selective estrogen receptor modulators may be a viable alternative to estrogens in the treatment of ischemic stroke. After experimental ischemia, there is already evidence that SERMs imitate the effect of estradiol, minimizing hormone‐dependent concerns. Tamoxifen substantially decreased infarct size and protected neurons from ischemia in rats treated to permanent MCAO (pMCAO). ⁸⁵ Recently, a novel type of estrogen receptor G protein‐coupled estrogen receptor (GPER, formerly known as GPR30) broadly distributed throughout the brain might possibly play a crucial role in estrogen‐mediated neuroprotective benefits in pathologies like stroke. ⁸⁶ This study reported that the GPER can be taken as a potential therapeutic target for stroke, mainly in males, as they cannot be exposed to estrogen therapy. ⁸⁶

6. PRECLINICAL AND CLINICAL STUDIES

Various preclinical studies carried out in the past few years have reported a promising role of XRs as a novel therapeutic target for stroke. Xenobiotic‐mediated induction of the Nrf2 signaling pathway is reported as a potential therapeutic strategy that may render long‐term neuroprotection. ⁵¹ However, rigorous clinical studies are needed to unravel robust mechanisms focusing on the Nrf2‐targeted interventions. A metagenomic study of different microbiota families using Phylogenetic Investigation of Communities by Reconstruction of Unobserved States (PICRUSt) software predicted that poststroke xenobiotic exposure can result in larger infarct size. ⁸⁷ When PICRUSt was used to predict post‐stroke alteration in the mucosal (gastrointestinal tract) microbiota in mice, it demonstrated enhanced involvement with infections along with xenobiotic degradation. ³⁴ Although any pathogen has not been clinically identified or any antibiotic has been established by clinical studies to specifically treat post‐stroke infections, bacterial pneumonia is one of the serious consequences of stroke. ⁸⁸ A clinical trial, STROKE‐INF, based on patients older than 18 years suffering from poststroke dysphagia concluded that antibiotic prophylaxis is ineffective for poststroke pneumonia prevention. ⁸⁸ Consecutively, preventive antibiotics in a stroke study also failed to establish third‐generation cephalosporin ceftriaxone as preventive antibiotics in adults with acute stroke, concluding the fact that parenteral antibiotics are not enough to prevent pneumonia in stroke. ⁸⁹

Several genetic variations of xenobiotic transporters are reported as a common risk factor for both ischemic stroke and coronary heart disease. According to a metanalysis study, a SNP (Val12Met) in ABCG2 (encoding a sterol and xenobiotic transporter) gene may be a common link between coronary heart disease and ischemic stroke. ⁹⁰ Pharmacological inhibitors of the BBB efflux transporters, such as P‐glycoprotein inhibitors, were subjected to a clinical trial, which allowed increased drug penetration into the brain. However, the study was a failure due to its higher systemic toxicity. ⁹¹ Substantial clinical studies on XRs reported an important role of xenobiotics as an important risk factor of stroke. Some important preclinical and clinical studies related to various aspects of xenobiotics and their receptors in stroke are tabulated in Table 2.

Table 2.

Preclinical and clinical trials on xenobiotics in stroke

Sl. No.	Purpose	Target	Animal model/No. of patients	Reference
Pre clinical studies
1	Activation of Nrf2 attenuates the brain damage	Nrf2	MCAO/rat	Liu et al. 51
2	Attenuation of oxidative stress by blocking P‐glycoprotein	P‐glycoprotein	129/SV/WT mice	Brzica et al. 92
3	Preventing BBB dysfunction by blocking P‐glycoprotein	P‐glycoprotein	MCAO/rat	Davis et al. 91
4	Inhibiting apoptosis and mediation of ROS generation	PPAR	MCAO/mice	Wnuk and Kajta 37
Clinical trials
1	Preliminary biomarker for platelet function in post‐stroke clopidogrel therapy	Rs13059232	634	Chen et al. 70
2	Increase in cerebral infarction risk in stroke patients with increase in CYP1A1c genotype	CYP1A1 gene 3′‐flanking regions (T6235C)	353	Moon et al. 78
3	Increase in cerebral infarction risk in stroke patients with increase in CYP1A1c genotype	CYP1A1 gene 3′‐flanking regions (T6235C)	215 (South Indian population)	Sultana et al. 79
4	Increase in cerebral infarction risk in stroke patients with increase in CYP1A1c genotype	CYP1A1 gene 3′‐flanking regions (T6235C)	1162 (Chinese population)	Zhang et al. 80

Abbreviations: BBB, blood–brain barrier; PPAR, peroxisome proliferator‐activated receptor; ROS, reactive oxygen species.

7. CONCLUSION AND FUTURE PROSPECTS

Regardless of current treatments for ischemic stroke in the form of thrombolysis and mechanical thrombectomy, there are still requirements for novel therapeutic approaches that may render neuroprotection to an extended period. To date, investigations on XRs in the context of stroke have limited reports. However, modulation of XRs may be a promising approach for stroke intervention. Among all XRs, the AhR is well reported as it is widely present in the CNS and participates in various neurological regulations. Therefore, extensive study is required to unravel other novel XR‐mediated signaling pathways that may be implicated in stroke diagnosis and treatment. Robust study should also be carried out considering genetic variants, gender disparity, and age‐related pathophysiological changes in stroke to have a safer stroke therapy in future.

AUTHOR CONTRIBUTIONS

Aishika Datta: Writing – original draft (lead); Writing – review & editing (lead). Bijoyani Ghosh: Writing – review & editing (supporting). Deepaneeta Sarmah: Writing – review & editing (supporting). Antra Chaudhary: Writing – review & editing (supporting). Anupom Borah: Writing – review & editing (supporting). Pallab Bhattacharya: Writing – original draft (lead); Writing – review & editing (supporting). All authors have read and agreed to the final version of the manuscript.

CONFLICT OF INTEREST

The authors declare no conflict of interest.

ETHICS STATEMENT

No animal or human studies were performed.

Datta A, Ghosh B, Sarmah D, Chaudhary A, Borah A, Bhattacharya P. Aspects of xenobiotics and their receptors in stroke. Neuroprotection. 2023;1:23‐34. 10.1002/nep3.9

DATA AVAILABILITY STATEMENT

Data sharing is not applicable to this article as no new data were created or analyzed in this study.