Mol Divers (2014) 18:895–909DOI 10.1007/s11030-014-9550-6

EXPERT OPINION

Pregnane X Receptor and P-glycoprotein: a connexionfor Alzheimer’s disease management

Sumit Jain · Vijay Rathod · Rameshwar Prajapati ·Prajwal P. Nandekar · Abhay T. Sangamwar

Received: 14 February 2014 / Accepted: 28 August 2014 / Published online: 12 September 2014© Springer International Publishing Switzerland 2014

Abstract The translational failure between preclinical ani-mal models and clinical outcome has alarmed us to searchfor a new strategy in the treatment of Alzheimer’s disease(AD). Interlink between Pregnane X Receptor (PXR) andP-glycoprotein (Pgp) at the blood brain barrier (BBB) hasraised hope toward a new disease modifying therapy in AD.Pgp is a major efflux transporter for beta amyloid (Aβ) athuman BBB. A literature survey reveals diminished expres-sion of Pgp transporter at the BBB in AD patients. Pregnane XReceptor is a major transcriptional regulator of Pgp. Restora-tion of Pgp at the BBB enhances the elimination of the Aβ

from brain alongside and inhibits the apical to basolateralmovement of Aβ from the circulatory blood. This reviewconcentrates on in vitro, in vivo, and in silico advancementson the study of the PXR in context to Pgp and discusses thesubstrate and inhibitor specificity between PXR and Pgp.

Keywords Pregnane X Receptor (PXR) · Alzheimer’sdisease · Amyloid beta (Aβ) · Blood brain barrier (BBB) ·P-glycoprotein (Pgp) · Neuroinflammation · In silico

Electronic supplementary material The online version of thisarticle (doi:10.1007/s11030-014-9550-6) contains supplementarymaterial, which is available to authorized users.

Sumit Jain, Vijay Rathod and Rameshwar Prajapati have contributedequally to this study.

S. Jain · V. Rathod · R. Prajapati · P. P. Nandekar ·A. T. Sangamwar (B)Department of Pharmacoinformatics, National Institute ofPharmaceutical Education and Research (NIPER),S.A.S. Nagar, Sector 67, Mohali 160062, Punjab, Indiae-mail: abhays@niper.ac.in

Introduction

Alzheimer’s disease (AD), discovered by Dr. Alois Alzheimerin 1907 [1], is a neurodegenerative disorder categorized bydementia owing to the deposition of extracellular amyloidplaques and intraneuronal neurofibrillary tangles. AD is nowtermed as the “disease of the twenty-first century.” Currentstatistics reveal the severity of AD, with over 36 millionpeople suffering from this neurodegerative disorder world-wide. In the United States (U. S. A.), almost 5.4 millionpatients live with AD being the sixth leading cause of deathamong all ages, which climbs to fifth in the case of elderlypatients (65 years or older) [2]. The death rate due to AD hasshown a steep increment over the last decade in the U. S. A.(Fig. 1) [3]. The death rate due to AD is subsequently risingup to 39 %, while it declined tremendously for other diseases(Fig. 2) [2]. The expenditure share for the treatment of ADis about 170 billion annually in the U. S. A. [2]. Due to therise in the number of cases over the time, AD is approach-ing an epidemic stature and thus demands effective treatmentstrategy.

Currently, the strategies adopted for AD are basicallycategorized into symptomatic therapy or disease modify-ing therapy. Symptomatic therapy addresses cognitive andneuropsychiatric aspects of AD, and is prominently utilizingneurotransmitter modulators such as 5-hydroxytryptamineantagonists, acetyl cholinesterase (AChE) inhibitors, gammaamino butyric acid (GABA-B) antagonists, inverse GABAagonists, N-methyl-d-aspartate (NMDA) antagonists, cogni-tive enhancers, and L-type calcium channel blockers. Diseasemodifying therapy has added new therapeutic approaches,which mainly targets pathogenesis of AD, such as Aβ pep-tide vaccination, secretases inhibitors, cholesterol-loweringdrugs, metal chelators, and anti-inflammatory agents(Table 1). Clinical benefits of these available pharmacologi-

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Fig. 1 Death rate (%) for allages due to Alzheimer’s diseasein last decade in the U.S.A. [2]

Fig. 2 Change in death rate(%) among different causes ofdeath in the U.S.A. [3]. (Colorfigure Online)

cal treatments are indisputable although limited to temporaryand symptomatic support to cognitive functions. In addition,consistent failure of new therapeutic interventions in AD overthe last decade in Phase-II proof of concept and pivotal Phase-III clinical trials collectively supports the need for more reli-able and effective treatment strategies for AD [4,5].

Human Pregnane X Receptor (hPXR) is a member oforphan nuclear receptor subfamily known for wide varietyof substrates and act as the xenobiotic sensing receptor [6].Upon activation, hPXR enhances the expression of its targetprotein by transcriptional regulation at the different sites ofbody, mainly liver and BBB. Regulation of various trans-porters mainly Pgp through PXR has recently shown theeffective way of improving central nervous system (CNS)pharmacotherapy and management of Aβ burden in AD [7].

PXR is also involved in the turnover of Apolipoprotein E(ApoE), which facilitates the disposition of Aβ outside thebrain. hPXR, along with other nuclear receptors, shows sig-nificant anti-inflammatory response, which replicates its con-trol over inflammation intracerebrally (Fig. 3). Altogether,PXR is implicated as an emerging target for AD in manag-ing Aβ overload and related complications.

Pathophysiology of Alzheimer’s disease

Dementia is a vast term covering several diseases and disor-ders related to malfunctioning and death of nerve cells (neu-rons) in the brain, leading to impaired memory, thinking,behavior, mood orientation, aphasia, and way of living. ADis characterized as the most witnessed dementia especially

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898 Mol Divers (2014) 18:895–909

Fig. 3 PXR-mediated interplay of various efflux transporters at BBBand their role in Alzheimer’s disease pathophysiology. Ligand bind PXRhetrodimerize with RXR act on the DNA which generates mRNA tran-

scriptionally induces various efflux transporters at blood brain barrier(right). Regulation of various inflammatory mediators by PXR (left).(Color figure Online)

in elderly patients and proves lethal with weakened essen-tial body functions such as locomotion, eating, and swallow-ing. The neuropathological and clinical examination revealedthat AD is characterized by the presence of amyloid plaques,and neurofibrillary tangles with cerebral amyloid angiopathy(CAA) and glial response [8–10]. Amyloid plaques mainlyconsist of 40–42 amino acid residues long amyloid β-peptide(Aβ) derived from β-amyloid precursor protein (APP) aftera series of cleavage catalyzed by enzymes such as betaand gamma secretases [11]. Aβ mainly accumulates in theparenchyma of the brain including areas of cortex and hip-pocampus [12,13]. Genetic evidence suggests that the muta-tion in the APP encoding gene results into the overproductionof Aβ by the action of beta and gamma secretases [14–16]. Aβ

plaques, in association with the microglial response, lead tothe generation of several cytotoxic factors including inflam-matory cytokines, interferon gamma (INFγ), tissue necroticfactor alpha (TNFα), interleukin 1β (IL-1β) and interleukin6 (IL-6) and chemokines (CCL2) [17,18]. Toxic microenvi-ronment, Aβ plaques, and neurofibrillary tangles (tau) col-lectively contribute to AD and neuronal loss. Hence, muchof the focus is laid on the elimination or reduction of Aβ load

from the brain as a therapeutic approach in the managementof AD.

Mechanism of Pregnane X Receptor

PXR is also known as a Steroid X Receptor (SXR) [19].While it is located abundantly in the liver and intestine, it isalso found in other parts of the body, such as brain, kidney,and lungs. Other nuclear receptors co-expressed with PXRare Constitutive Androstane Receptor (CAR), Aryl Hydro-carbon Receptor (AhR), Liver X Receptor (LXR), FarnesoidX Receptor (FXR), and Vitamin D Receptor (VDR). PXRwas originally identified on the basis of its sequence homol-ogy with other nuclear receptors [20,21]. In 1997, a mousesequence appeared in sequence tag database maintained byWashington University, characterized as a novel NR LBD(nuclear receptor ligand binding domain) fragment, whichlater translated into the complete protein. A cDNA encrypt-ing the complete mouse protein was successively cloned, andthe receptor was named as PXR based upon its activation bya wide variety of natural and synthetic pregnanes. Human

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Fig. 4 Mechanism of activity of PXR enhancing expression of efflux transporter in the various parts of the body. (Color figure Online)

PXR was cloned by three separate groups and, therefore, itis also referred as the Pregnane-Activated Receptor (PAR) orSXR, broadly coined as hPXR [22,23].

The activity of PXR is regulated by many of bio-moleculesand ligands namely, co-repressors, co-activators, induc-ers/agonists, inhibitors/antagonists, and endogenous ligands.In the absence of agonist, PXR complexes with the co-repressors such as NCoR1 and NCoR2 (nuclear receptor co-repressor 1 and 2), which checks the transcriptional potentialof PXR by the involvement of histone deacetylase. When anagonist binds to PXR, it induces conformational changes inthe PXR structure resulting in the breakage of co-repressor–PXR complex and formation of new co-activator–PXR com-plex, with co-activator such as SRC1 and SRC3 (steroidreceptor co-activator 1 and 3). These co-activator–PXR com-plexes possess intrinsic histone acetyltransferase activityresulting into chromatin rearrangement and, in turn, modifythe transcriptional activity of ligand bound PXR complex.The complex then combines with its heterodimer partnerRetinoid X Receptor-Alpha (RXRα), and heterodimeriza-tion acts on the DNA to produce mRNA for the transcrip-tional activation of the target genes for cytochrome P450enzymes (CYPs) and ATP binding cassette protein (ABC)efflux transporters, resulting in the over-expression of Pgpand other transporters (Fig. 4) [6,24].

Pregnane X Receptor and ABC transporters inAlzheimer’s disease

Over the last decade, researchers have been trying to estab-lish the role of ABC transporters in the pathophysiology ofAD and its complications [25]. ABC transporters are thesuperfamily of efflux transporters which utilize ATP to trans-port their substrates across the cell membrane and the BBB.ABC efflux transporters cover a wide variety of endoge-nous and exogenous substances as their substrates and areinvolved in the clearance of the toxic metabolites from differ-ent cells, tissues, and organelles. ABC transporters locatedaround the BBB create a barrier for many xenobiotics andalso clear their toxic metabolites from the brain [25]. Thetype of ABC transporters expressed in the vicinity of theBBB include Pgp (ABCB1/MDR1), BCRP (ABCG2), andvarious MRPs 1–5 (ABCC1-5) and CERP (ABCA1) (Table2) [26,27]. BBB accommodates PXR around blood capillar-ies that are involved in the regulation of the efflux transporters[28]. Efflux transporters that are transcriptionally regulatedby PXR include organic anion-transporting polypeptide iso-form 2 (SLCO1A4), bile salt export pump (ABCB11), andPgp [22,29,30]. The literature reveals that the Aβ is a sub-strate for various ABC transporters [31]. Pathophysiology ofAD is related to weak Pgp expression around BBB,which

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Table 2 Various ABC transporters at blood brain barrier and their characteristics [25,65,66]

Name Synonyms Subfamilyclassification

Direction Location Substrate

Endobiotics Xenobiotics

P-glycoprotein Multidrugresistanceprotein(MDR1)

ABCB1 Basolateral toapical

Apical BBB choroidplexus (sub apical)

Amyloid beta Aβ),steroid hormones,cholesterol,prenylcystein-methylesters, IL-2, IL-4,IFN-γ and toxins

Vinblastine,cyclosporin A,digoxin,antidepressant,antipsychotic,antiepilepticdrugs

Multidrugresistance-associatedprotein

Canalicularmultispecificorganic aniontransportergene (CMOAT)

ABCC1-5 Basolateral Toapical andapical tobasolateral

Luminal and abluminalBBB choroid plexusbasolateral membrane,astrocytes, pericytes,neurons, and microglia

– Estradiolglucuronide

Breast cancerresistanceprotein

Mitoxantroneresistanceproteins(MXRP) andABCP

ABCG2 Basolateral toapical

Luminal BBB astrocytes,pericytes, neurons braincapillary endothelial cells

– Prazosin,mitoxantron

Cholesteroleffluxregulatoryprotein

– ABCA1-2 Basolateral toapical

Choroid plexus, neurons,astrocytes, microglia,oligodendrocytes, andbrain capillary endothelialcells

Amyloid beta (Aβ),cholesterol,phospholipid, highdensity lipoprotein

can be exploited for elimination of Aβ out of the brain bystimulating Pgp expression through PXR.

P-glycoprotein (Pgp) transporter

Pgp efflux transporter is well known among ABC trans-porters for its involvement in multidrug resistance and drug–drug interactions with many drugs. Various in vitro and invivo studies revealed that Pgp around the BBB regulates thetransport of endo-xenobiotics across the encephalon [32,33].Pgp also mediates the removal of toxins and reactive metabo-lites from the brain, since the high expression of Pgp isobserved in the luminal side of the endothelial cells [34].A similar study provided evidence that Pgp is involved in theclearance of Aβ from the brain into blood [35]. In anotherstudy, Pgp deficiency at the BBB increased Aβ deposition inan mdr1a/b double knockdown (Pgp null) mouse model. Fur-thermore, after administration of a Pgp inhibitor, the steeprise in Aβ level was observed in the brain interstitial fluid(ISF) [36]. There is an inverse relationship between Pgp andAβ in terms of expression. This validates the role of Pgp in theturnover of Aβ inside the intracerebral space. A circulatoryvessel with high Pgp expression showed no accumulation ofAβ, whereas Aβ deposition was detected in vessels with lowPgp expression. Recently, it has been observed that the Aβ

down regulates the Pgp expression in the vicinity of the BBB,

which diminishes the Aβ clearance from brain and exaggerateAD [37]. Circulating Aβ tends to decrease Pgp in the capil-laries which induces upregulation of Pgp in the vessels as anegative feedback mechanism to compensate for the loss ofPgp and enhances frequently generating Aβ clearance fromthe brain [38]. Later on, upregulated Pgp was lost resultingin the accumulation of Aβ in arteries, arterioles, capillaries,and vessels leading to CAA and finally into AD [38]. Theexpression of Pgp is regulated by various nuclear receptors,mainly PXR, and a variety of xenobiotics [39]. The PXR ago-nists pregnenolone-16α-carbonitrile (PCN) and dexametha-sone enhance the Pgp expression in the rat brain capillaries[28]. Further, in vitro and in vivo experiments using rifampinand hyperforin in the hPXR transgenic mice model revealedthat elevated Pgp levels in brain capillaries subsequentlytighten the BBB and restrict entry of methadone into the brain[40]. Similar results were obtained in a pig model utilizing pigbrain capillary endothelial cell expressing pig PXR (PgPXR).Rifampin, hyperforin, and PCN were used as PXR agonists.Human PXR ligands, rifampicin and hyperforin, have acti-vated PgPXR at BBB, and subsequently elevated Pgp expres-sion, whereas murine PXR ligand PCN was unable to stimu-late PgPXR [41]. Restoration of BBB Pgp has reduced intrac-erebral Aβ in male transgenic human APP-over-expressingmice using PCN-mediated PXR activation [7]. Collectively,upregulation of Pgp in the vicinity of the BBB can be usedas a novel approach toward the management of AD.

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Table 3 Common PXR inhibitors and Pgp substrate and inhibitors[67–131]

PXR inhibitors Pgp inhibitors PXR inhibitors Pgp substrates

Coumestrol Y Camptothecin Y

Fucoxanthin Y Fluconazole Y

Itraconazole Y Itraconazole Y

Isosilybin Y Ketoconazole Y

Ketoconazole Y Leflunamide Y

Sesamin Y Stigmasterol Y

Stigmasterol Y

Y yes

PXR and Pgp: a cross-talk within similarities

Inducers/activators of PXR are increasingly investigated forcontrolling AD [7]. A plethora of data is published reportingnovel PXR activators, accountable for the regulation of Aβ.The accumulation of Aβ leads to inflammation at the BBB,which results in the worsening of AD. The activation of PXRis responsible for the induction of the transcription factor(TF), which in turn regulates various transporters, mainlyPgp. This suggests that PXR activators have a positive feed-back on expression of ABC transporters. Here, we reviewedmost of the potential PXR activators and their possible impacton expression of Pgp. Literature suggest that PXR activa-tors manifest favorable effect whereas, PXR inhibitors haveshown inverse effect on the AD. It has been found that theblockade of Pgp results in the increased level of the Aβ, fur-ther increasing the chances of inflammation in the brain, andultimately aggravates the AD condition. Tables 3 and 4 enlistthe drugs that act as activators and inhibitors of PXR. Thecritical review of activators and inhibitors of PXR (enlistedin Tables 3, 4) noticed that PXR inhibitors/activators may actas Pgp substrates/inhibitors.

Structural similarity between PXR and Pgp: in silicoinsights

Herein, we reviewed the structural details of PXR and Pgp fortheir structural similarities, particularly drug binding site(s).Both PXR and Pgp show substrate promiscuity, binding to adiverse class of chemically unrelated drugs. PXR has shownbinding promiscuity with substrates/activators as small asphenobarbital (232 Da) and as large as rifampicin (823 Da)and taxol (854 Da) [42]. Similarly, Pgp functions as anATP-driven efflux pump for substrates ranging from approx-imately 300 to 4,000 Da in mass, for example it allowsin a small molecule such as mitomycin (molecular weight334 Da) and a large drug molecule such as cyclosporine(molecular weight 1,202 Da) [43–45]. This suggests that both

Table 4 Common PXR activators and Pgp substrate and inhibitors[67–131]

PXRactivators

Pgpsubstrate

PXRactivators

Pgpinhibitor

St. John wart Y St. John wart Y

Rifampin Y Praeruptorin D Y

Paclitaxel Y 1-Piperoylpiperidine Y

Amprenavir Y Hyperforin Y

Carbamazepine Y Erlotinib Y

Corticosterone Y Clotrimazol Y

Dexamethasone Y Indomethacin Y

Phenobarbital Y Nifedipine Y

Vincristine Y Tamoxifen Y

Clotrimazole Y Schisandrin A Y

Verapamil Y Lovastatin Y

Lovastatin Y Mifepristone Y

Bisphenol A Y Pregnanedione Y

Cerivastatin Y 4-Hydroxtamoxifen Y

Clindamycin Y Hyperforin Y

Dicloxacillin Y Isradipine Y

Docetaxel Y Miconazole Y

Efavirenz Y Nicardipine Y

Endosulfan Y Progesterone Y

Erlotinib Y 17-OH-progesterone Y

Fluvastatin Y Santonin Y

Fenvalerate Y Crypterone acetate Y

Dehydroepiandrosterone Y DDT Y

17β-Estradiol Y Verapamil Y

Vitamin K2 Y Nicardipine Y

Atorvastatin Y Reserpin Y

Chlorpyrifos Y Atorvastatin Y

Warfarin Y Dihydrotestosterone Y

Cryptotanshinone Y Pentachlorophenol Y

Cypermethrin Y Zolpidem N

Y yes, N no

PXR and Pgp have wide range of substrate binding affinityand get activated by wide range of drug molecules.

hPXR has been extensively studied using X-ray crys-tallography. Eleven hPXR crystallographic structures areavailable in the Protein Data Bank (PDB). These structuresare available in different forms: apo form (without ligand)(PDB-ID: 1ILG) [46], in complex with co-crystallized lig-ands SR12813 (PDB-ID: 1ILH) [46] and hyperforin (PDB-ID: 1M13) [47], and in complex with SRC-1 peptide alongwith co-crystallized SR12813 (PDB-ID: 1NRL) [48]. Thecanonical structure of hPXR ligand binding domain (LBD)consists of seven α-helices, arranged in three layers sand-wiching the LBD. The three layers are arranged in asequence α 1, α 3, α 4, α 5, α8, and α 7, α10. Unlike othernuclear receptors, hPXR has five extended stranded antipar-

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Fig. 5 Canonical structure of hPXR ligand-binding domain (LBD) andits structural topology showing arrangement of alpha helixes, beta sheet,and loops. (Color figure Online)

allel β-sheets (β 1, β 2, β 3, β 4, and β 1′); β1 and β 1′ that areunique to human PXR (Fig. 5) [42].

In contrast, no human Pgp crystal structure is yet avail-able. The crystallographic structures of Pgp available inPDB are those from different organisms with low reso-lutions. Sav1866 (Staphylococcus aureus transporter) andMsbA Pgp crystallographic structures were the initial struc-tures reported with better resolution compared to other avail-able structures [49,50]. The MsbA crystallographic structurewas reported in outward-open conformation. The Sav1866crystallographic structure was an outward-facing conforma-tion of Pgp obtained in the presence of ATP with NBDs inclose proximity. Other significant crystal structures of Pgpwere published by Aller et. al. include PDB IDs: 3G5U,3G60 and 3G61 [51]. Out of these crystallographic struc-

tures, 3G5U was reported with a resolution of 3.80 Å and87 % sequence identity with human Pgp. However, certainregistered errors in the TM3, TM4, and TM5 regions werereported in the murine Pgp crystallographic structure in therecently published C. elegans Pgp crystal structure [52].

Pgp is a trans-membrane single polypeptide that is struc-turally composed of two homologous parts; each homologcontains six trans-membrane (TM) segments followed bya consensus nucleotide-binding domain (NBD). The twohomologous parts are separated by an intracellular linkerregion of about 60 amino acid residues. Figure 6 shows thehomology model of human Pgp generated using C. eleganscrystal structure as template in our previous study [53]. TheTM segments of Pgp are the putative site for substrate recog-nition and the NBDs are the site for ATP binding/hydrolysis.

Although PXR and Pgp have different structures, theyshare some structural similarities in the LBD. The drug bind-ing domains of both the proteins are highly hydrophobicand share many substrates in common (Tables 3, 4). Thehydrophobic cavity volume in PXR ranges from 1,294 to1,544 Å3, depending on the substrate or co-activator bindingto PXR [54–56]. A cavity volume of 1,294 Å3 is reported inapo crystallographic structure [53]. PXR crystal structure incomplex with the cholesterol lowering compound SR12813showed the hydrophobic cavity volume of 1,280 Å3 [53]. Acavity volume of 1,344 Å3 was observed in PXR structure incomplex with SR12813 and SCR-1 co-activator [54], and acavity volume of 1,544 Å3 in co-crystallized structure withhyperforin [55]. This clearly indicates that the PXR LBDis highly flexible to accommodate compounds of varyingsize and molecular mass. However, the cavity volume of Pgp

Fig. 6 In house developed homology model of human P-gp and arrangement of TM segments. (Color figure Online)

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Fig. 7 Electrostatic surfacepotential representation of PXRstructure. a Two possiblesubstrate gates (encircled)noticed in PXR crystal structureand the Leu residue controllingthe access of substrate throughthese gates. b The three loopsgoverning access to thehydrophobic cavity, substrateaccessibility depends on theflexibility of residues present onthese loops, Leu209 occupiesthe central position and isbelieved to be gating thesolvent-accessible channel.(Color figure Online)

varies dramatically during the course of transport cycle. Pgpcan accommodate two or more compounds simultaneouslywithin the internal cavity due to its large volume of ∼6,000Å3

[51,57]. The volume of this internal cavity is reduced gradu-ally as NBDs approach each other, ultimately reverting whenNBD dimerizes, exposing the drug binding region to extra-cellular space.

Access to the hydrophobic cavity is gated by a hydropho-bic channel in both PXR and Pgp. In PXR, the access tothe hydrophobic cavity is governed by loops joining α2 toβ1, α3 to β 1′, and α6 to β4. The access to the hydrophobiccavity depends on the flexibility of the residues present onthese loops, and in particular on the restructuring of α2 helixthat opens a solvent-accessible channel gated by Leu209(Fig. 7). This channel is over 9 Å in length and stretchesalong the loops joining α3 to β 1′ and α2 to β1, and is solvent-accessible, 3 Å wide [55].

However, in Pgp the access to the drug binding region israther bewildering. Three models for Pgp substrate transloca-tion are suggested: (1) pore, (2) flippase, and (3) hydrophobicvacuum cleaner [58,59]. The hydrophobic vacuum cleanermodel is most acceptable around scientific community, asrecent Pgp crystal structures show the presence of hydropho-bic portals in the TMD region nexus lipid bilayer. Aller etal. reported two portals that allow access to the entry ofhydrophobic molecules directly from the membrane [51].The portals are formed by TMs 4/6 and 10/12 (Fig. 8). Theportals formed by intertwining these TM helices are ∼9 Åwide at the widest point.

Figure 9 shows an electrostatic potential surface compar-ison of PXR and Pgp labeling the residues essential for sub-strate binding. The SR12813 ligand mostly interacts throughhydrophobic interactions at the active site of PXR, forminghydrophobic contacts with side chain of 11 amino acids liningthe PXR ligand-binding cavity (Fig. 9). Further, π -stackinginteraction between the aromatic indole ring of Trp299 and

O11 hydroxyl oxygen atom of SR12813, and H-bondinginteractions with Ser247 and His407 were also observed. Itis important to note that these interactions are not observedin the previous structure [42,54].

These structural similarities between PXR and Pgp alongwith the abundance of common ligands manifest beneficialeffect in the AD pathophysiology with respect to the effluxof intracerebral Aβ, a major cause of AD and its associatedneuroinflammation. Structural similarities mentioned abovereveal the reason behind the cross-talk between PXR and Pgpligands since both share hydrophobic binding site and suffi-cient degree of structural similarity resulting in the large num-ber of common ligands able to bind and activate both PXRand Pgp simultaneously. Further, PXR activation results inthe up-regulation of Pgp along with the BBB, which results ina synergistic effect in the efflux of intracerebral Aβ plaques.Exploiting this connexion between PXR and Pgp may proveeffective in AD management.

Role of PXR in neuroinflammation of Alzheimer’sdisease

Inflammation is the central component of AD pathophys-iology, which increases the neurodegeneration and deathof neuronal cells. Neuroinflammation is the successiveeffect of accumulation of a variety of substances in theperiphery of neurons, which includes Aβ neuritic plaques,tau-neurofibrillary tangles, neurofilament, oligodendrocytemyelin glycoprotein, extracellularly exposed DNA, andbyproduct of damaged neurons [60]. Deposition of Aβ andother proteins, such as tau and intraneuronal neurofibrillarytangles, results in increased expression of microglia and othermacrophages. Accumulation of microglia and macrophageswith Aβ and tau triggers various complementary and alter-native signaling cascades resulting in the generation of

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Fig. 8 Electrostatic surfacepotential representation of P-gpstructure showing thehydrophobic gates noticed inP-gp structure. These gates arelocated in the membraneembedded region of P-gp andformed by TMs 4/6 and 10/12.(Color figure Online)

Fig. 9 Electrostatic surface potential representation of substrate bind-ing cavity of P-gp and PXR showing the arrangement of hydrophobicresidues in stick representation. a Hydrophobic substrate binding cavityof PXR showing the bound co-crystallized structure (orange) and thearrangement of hydrophobic residue that binds the substrates through

hydrophobic interactions. b, c Vertical sections of P-gp structure show-ing hydrophobic substrate binding region in P-gp and arrangement ofhydrophobic residues. Hydrophobic cavity of P-gp is relatively verylarge as compared to PXR. (Color figure Online)

pro-inflammatory mediators [61]. In addition to microglia,astrocytes also concentrate around Aβ deposits and neu-ritic plaques, which induce the production of the inflamma-tory regulator, such as NF-κB (nuclear factor-κB) and COX(cyclooxygenase). Furthermore, neurons themselves are the

producer of the neurotoxic milieu by enhancing inflamma-tory proteins. The inflammatory network comprises NF-κB,COX, β2-integrin, cytokines such as IL-1β and IL-6, TNFα,TGF β1, β2, β3 (transforming growth factor - β1, β2, β3), var-ious chemokines α (CXC), β (CC), γ (C), δ (CX3C), α1- ACT

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(α1-antichymotrypsin), α2 -MAC (α2 - macroglobulin), C5b-9 MAC (membrane attack complex), M-CSF (macrophage-colony stimulating factor) prostaglandins, iNOS (induciblenitric oxide synthase), and oxygen-free radicals [62]. Further,chronic inflammation has a negative impact on efflux trans-porters mainly Pgp, drug metabolizing enzymes (DMEs),and their transcriptional regulatory machinery resulting inthe depletion of Pgp and cytochrome-P450 metabolizingenzymes. Collectively, these inflammatory mediators areresponsible for neurodegeneration and neuronal cell death.

Inflammation enhances the neurodegeneration and neu-ronal death. In this aspect, NSAIDS are included in the ADpharmacotherapy. Activated PXR is known to reduce pro-inflammatory mediators in the intestinal bowel syndrome(IBD) by interfering with the NF-κB regulated inflammatorysignals [63]. PXR-mediated neuroprotection was observedin the case of the Neimann Pick C (NPC) disease utilizingallopregnanolone which activates PXR in vivo [64,65]. PXRis involved in the prominent cross-talk with PPAR, LXR, andRXRα which possess substantial anti-inflammatory responsein ligand-activated manner by functionally suppressing NF-κB inflammatory response in various AD experimental mod-els [19].

Hurdles for treating Alzheimer’s disease with PregnaneX Receptor

Various therapies are implicated in the treatment of AD, butnone could give a complete solution for the treatment ofAD due to their own limitations. A newly proposed ther-apy involves targeting the regulatory machinery of Pgp,which improved the AD pathophysiology by clearing Aβ bur-den intracerebrally. Regulatory machinery consists of PXRthat transcriptionally regulates the various DMEs and trans-porters in different parts of the body, prominently liver, intes-tine, and BBB. Recently, a study using a mouse AD modelhas shown that restoration of Pgp in the BBB by activat-ing PXR reduces the Aβ overload [7]. Thus, targeting Pgpvia PXR has a potential role in Aβ clearance from the brainand attenuates Aβ accumulation intracerebrally. This newapproach suggests a new therapeutic strategy in the AD. Fur-thermore, species-specific drug response is observed in thecase of PXR since enough species diversity exists among rat,mouse, rabbit, and human PXR where a drug shows differ-ent responses for PXR activation/deactivation. One drug mayactivate human PXR and no effect on rat PXR, and vice-versa.

Conclusions

Aβ accumulation accompanied with inflammation is one ofthe major contributing factors for AD-associated neurode-

generation and neuronal cell death leading to dementia andimpaired memory. Clearance of Aβ outside the brain emergesout as one of the therapeutic strategies to manage AD. PXR,as a nuclear receptor, shows promising results in transcrip-tional regulation of efflux transporters mainly Pgp at theBBB. Overexpression of the efflux transporters at the BBB ismediated by PXR, which reduces Aβ burden intracerebrally.Additionally, PXR controls ApoE and inflammation offeringa further advantage to its role in the management of AD.Collectively, we propose PXR as an attractive target for ADmanagement.

Acknowledgments This work was supported by the Departmentof Biotechnology (DBT), India.

Conflict of interest The author declares no competing financialinterest.

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